Your cluster implements HDFS High Availability (HA). You two NameNodes are named nn01 and
nn02. What occurs when you execute the command:
Hdfs haadmin -failover nn01 nn02

A.
nn02 becomes the standby NameNode and nn02 becomes the active NameNode

B.
Nn01 is fenced, and nn01 becomes the active NameNode

C.
Nn01 is fenced, and nn02 becomes the active NameNode

D.
Nn01 becomes the standby NameNode and nn02 becomes the active NameNode

Explanation:
failover – initiate a failover between two NameNodes
This subcommand causes a failover from the first provided NameNode to the second. If the first
NameNode is in the Standby state, this command simply transitions the second to the Active state
without error. If the first NameNode is in the Active state, an attempt will be made to gracefully
transition it to the Standby state. If this fails, the fencing methods (as configured by
dfs.ha.fencing.methods) will be attempted in order until one of the methods succeeds. Only after
this process will the second NameNode be transitioned to the Active state. If no fencing method
succeeds, the second NameNode will not be transitioned to the Active state, and an error will be
returned.
HDFS High Availability Administration, HA Administration using the haadmin command