In the following question, various terms of a letter series are given with one term missing as shown by (?). Choose the missing term out of the given alternatives.

If we observe series the first term of series follows the patterns like +3,+4,+3,+4.... while the second terms follow likes +4,+3,+4,+3.
D+3=G
G+4=K
K+3=N
N+4=R
R+3=U
similarly
F+4=J
J+3=M
M+4=Q
Q+3=T
T+4=X
So correct terms is UX.