MongoDB C100DBA - MongoDB Associate Database Administrator(Based on MongoDB 6.0 and 7.0+)

replaceOne() replaces the matched document ' s content with the replacement document, except that the immutable _id value is preserved when it is not explicitly changed. Ordinary application fields that are omitted from the replacement disappear. The required final document must retain userid, retain first, and change roles to " SuperUser " . Option B supplies all three fields and therefore produces the requested document structure. Option A omits userid, so that ordinary field would be removed from the replacement. Option C omits both userid and first, leaving only roles in addition to the preserved _id. Option D incorrectly uses the update operator $set inside a replacement document. $set belongs with update operations such as updateOne() or updateMany(); a replaceOne() replacement must be a replacement document rather than an update-operator document. This distinction is important for DBAs and application developers because accidental use of a partial replacement can delete fields that were intended to remain. If the goal is only to change one field, updateOne({userid:1}, {$set:{roles: " SuperUser " }}) would normally be safer. Given the choices, B is correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - replaceOne(), full-document replacement, and update-versus-replacement semantics.

===========

The supported mongosh helpers for replica-set membership changes are rs.remove() and rs.add(). rs.remove(hostname) removes the member whose host string matches the supplied host:port, while rs.add() adds a new member using either a host string or a member configuration document that includes a host field. Option C uses those documented methods in the required sequence: remove node 1, remove node 2, then add node 1 back. The names rs.removeNode() and rs.addNode() shown in the other options are not standard MongoDB replica-set helper methods. Membership changes must be performed carefully because modifying voting or data-bearing membership can affect majority calculations, elections, and availability. Administrators normally execute membership changes while connected to the current primary, allow configuration propagation, and verify replica-set health afterward with rs.status() or related diagnostics. If a removed data-bearing node is later re-added, it may need to synchronize depending on how far its data has diverged from the current replica-set history. Among the choices, option C is the only valid command sequence.

Study Guide reference/topic:Cluster Reliability and Data Resilience - replica-set membership, rs.remove(), rs.add(), and topology changes.

===========

The first argument to find() is the query filter and the second argument is the projection. Because the requirement applies to every employee, the filter must be an empty document {}. The projection must include name and age while explicitly suppressing _id. MongoDB includes _id by default even in inclusion projections, so { name: 1, age: 1 } would return three fields: _id, name, and age. Adding _id: 0 is the documented exception that allows _id to be excluded from an otherwise inclusion-style projection. Option C therefore produces exactly the requested fields. Options A and B mistakenly place projection-like expressions in the filter position, which would cause MongoDB to search for documents where those fields literally match the specified values rather than controlling returned fields. Option D is close but violates the word “ONLY” because _id remains present. Projection design can also reduce network traffic and, when a suitable index covers both the predicate and projected fields, can contribute to covered-query plans. For this requirement, however, the essential syntax is find({}, {name:1, age:1, _id:0}), so C is correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - find(), filters, projections, inclusion fields, and _id exclusion.

===========

db.collection.insertOne() inserts one BSON document into a collection. In mongosh, string values must be quoted and Boolean literals use JavaScript syntax: lowercase true or false. Therefore the student ' s name must appear as " Randall " , and the passed flag must be true. Option D is the only choice that gets both value types correct. Option A quotes the string properly but uses True, which is not the JavaScript Boolean literal. Options B and C leave Randall unquoted, so mongosh would interpret it as an identifier or variable rather than as the literal string required by the document. Option C additionally uses the invalid capitalized Boolean form. If an _id field is omitted, MongoDB automatically adds a unique _id, typically an ObjectId, before storing the document. That behavior does not affect the two requested fields. This question tests both CRUD syntax and the BSON/JavaScript literal representation used by mongosh. The correct command is db.class101.insertOne({ student: " Randall " , passed: true }), so option D is correct.

Study Guide reference/topic:MongoDB Architecture and Core Operations - insertOne(), BSON values, strings, Booleans, and automatic _id.

===========