Select the combination of letters that when sequentially placed in the gaps of the given letter series will complete the series. TD_GHS_KT_FG_ST_TD_GHS_K

FTDHKFT FDTHKFT FTDHKFT DHKFTTF

FTDHKFT

Dividing the given series into 3 parts of 8 letters each and comparing the corresponding terms we get the repetitive term as TDFGHSTK. TDFGHSTK / TDFGHSTK / TDFGHSTK So, the code will be FTDHKFT