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