Which option would you use to reduce fragmentation without affecting the users who are currently using the index?

While observing the index statistics, you find that an index is highly fragmented, thereby resulting in poor database performance.
Which option would you use to reduce fragmentation without affecting the users who are currently using the index?

A.
Deallocate the unused space in the index using the ALTER INDEX … DEALLOCATE UNUSED command

B.
Change the block space utilization parameters using the ALTER INDEX command

C.
Rebuild the index using the ALTER INDEX … REBUILD ONLINE command

D.
Validate the index structure using the ANALYZE … INDEX command