a) The amount of oplog to keep per backup is configurable and affects the time window available for checkpoint and point-in-time restores
b) To get a consistent snapshot of a sharded system, you must enable the balancer
c) Backups produced by copying the underlying data do support point in time recovery
d) All of the mentioned

Answer: a
Explanation: When you restore from a checkpoint or point in time, Ops Manager first creates a local restore of a snapshot from the blockstore and then applies stored oplogs until the specified point is reached.