How can you reduce fragmentation of an index without affecting the current transactions that are using the index?

How can you reduce fragmentation of an index without affecting the current transactions that are
using the index?

A.
Use the ANALYZE INDEX . . . command

B.
Use the ALTER INDEX . . . VALIDATE STRUCTURE command

C.
Us the ALTER INDEX . . . REBUILD ONLINE command

D.
Use the ALTER INDEX . . . DEALLOCATE UNUSED command

Explanation:
Use the deallocate_unused_clause to explicitly deallocate unused space at the end
of the index and make the freed space available for other segments in the tablespace.
If index is range-partitioned or hash-partitioned, then Oracle Database deallocates unused space
from each index partition. If index is a local index on a composite-partitioned table, then Oracle
Database deallocates unused space from each index subpartition.
Reference: Oracle Database SQL Language Reference 11g, alter index