Correct Answer: GTDAFVD

Solution : Given:
ASDF_A_ _FG_UD_GA_ _FG

To fill the series we have to divide the series – ASDF_ / A_ _FG / _UD_G / A_ _FG

Let's check each option –
First option: GTDAFVD; ASDF G / A TD FG / A UD F G / A VD FG (In each part, the second letter is increased by 1 place, and rest of the letters are the same)
Second option: FTEAFWD; ASDF F / A TE FG / A UDFG / A WD FG (No repetitive pattern is followed)
Third option: FTDAFVD; ASDF F / A TD FG / A UD F G / A VD FG (No repetitive pattern is followed)
Fourth option: GTEAGXE; ASDF G / A TE FG / A UD G G / A XE FG (No repetitive pattern is followed)

So, only the first option follows a pattern. Hence, the first option is correct.