MongoDB applies database operations on the primary and then records the operations on the primary's oplog. The secondary members then copy and apply these operations in an asynchronous process. All replica set members contain a copy of the oplog, in the local.oplog.rs collection, which allows them to maintain the current state of the database.
Reasoning behind the right answer:
The oplog (operations log) is a special capped collection that keeps a rolling record of all operations that modify the data stored in your databases.
Unlike other capped collections, the oplog can grow past its configured size limit to avoid deleting the majority commit point.