A.Rebuild the index using the online option.
B.Take the individual data file offline (not the whole tablespace), restore and recover the data file from backup with RMAN, then set the data file online again.
C.Without setting the individual data file offline, use RMAN with Block Media Recovery to restore and recover only those blocks.
D.Take the individual data file (not the whole tablespace) offline, use RMAN with Block Media Recovery to restore and recover only those blocks, then set the data file online again.