WU :FD :: ZX : ?

W is 23 rd letter from start and 26_25_24_23( 4 th letter____ from End) === 4 th letter is D
U is 21 st letter from start and 26_25_24_23_22_21( 6 th letter from End)== 6 th letter is F

so Z is 26 th letterfrom start __ 1 st letter from End ___ first letter is == A
X is 24 th letter from start___3 rd letter from End __ third letter is == C

so W(4) U(6) : F(6) D(4) :: Z(1) X(3) : C(3) A(1)