A MongoDB administrator is currently managing a single node MongoDB Enterprise deployment.

Adding replica-set members addresses availability and recovery objectives. A normal secondary maintains a replicated copy of the primary ' s data and can be elected primary if the current primary becomes unavailable, thereby improving service availability compared with a single-node deployment. That directly supports option D. A delayed member intentionally applies oplog entries later than the primary, preserving an older logical state of the dataset for a configured delay period. This can help recover from operator mistakes such as an accidental destructive update or drop operation because the administrator may be able to access the delayed copy before the damaging operation is applied there. That supports option A. A delayed member is not a substitute for long-term archival or a formal backup-retention policy; its recoverable history is constrained by the delay configuration and oplog window, so B is not the intended objective. Option C describes sharding, which partitions data across shards rather than simply replicating it. Encryption at rest is configured independently and is unrelated to adding secondaries or delayed members. Therefore, A and D are the two reasons that align with the proposed replication design.

Study Guide reference/topic:Cluster Reliability and Data Resilience - replica sets, delayed members, failover, and operator-error recovery.

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