On startup, the NameNode enters a special state called Safemode. Replication of data blocks does not occur
when the NameNode is in the Safemode state. The NameNode receives Heartbeat and Blockreport messages
from the DataNodes. Which of the following contains the list of data blocks that a DataNode is hosting?

A.
Rack Awareness

B.
Snapshot

C.
Replica

D.
Blockreport

Explanation:
https://hadoop.apache.org/docs/r1.2.1/hdfs_design.html