The given sequence consists of two series :
I.    Z, W, T, V, Q, ? in which each letter is moved three steps backward to obtain the next term.
II.   S, O, K, G in which each letter is moved four steps backward to obtain the next term.

The first letters are in alphabetical order with a letter skipped in between each segment: C, E, G, I, K. The second and third letters are repeated; they are also in order with a skipped letter: M, O, Q, S, U.

There are two alphabetical series here. The first series is with the first letters only: STUVW. The second series involves the remaining letters: CD, EF, GH, IJ, KL.

The letter of the word are written in a reverse order and each letter, except the first and the last one, is moved one step forward, to obtain the code

S      U       G       A        R
R     P         K       L         U

Similarly,

G       U     R        U       S
K        P     U        P       R

Z+3=C-2=A+3=D, X+3=A-2=Y+3=B, V+3=Y-2=W+3=Z,_______ T + 3 = W – 2 = U + 3 = X => TWUX

Z-2=X-2=V-1=> U-2=S-2=Q-1 => P-2=N-2=L-1=> K-2 = I-2 = G => KIG

Corresponding numbers of the letters are squared, and taken in alphabetical order.

Hence E = 52 = 25 => Y = EY

BCE, FGI, JKM, NOQ, …..

B+1-C+2-E,+1 => F+1=G+2=I,+1 => J+1=K+2=M,+1 => N+1=O+2=Q,+1 => R+1=S+2 = U

AB, CD, EF, GH are in alphabetical order and YZ, WX, UV are the corresponding letters of the alphabets in reverse order.

AM, BL, CK, DJ, => EI

First letter of the series in alphabetical order

From A Second letter is in reverse alphabetical order from M

In alphabetical order, middle letter is decreasing by 1 letter.

In the first letter of the every series is increasing by 1, middle letter is continuously decreasing by 1 [-1].

Third letter is increasing by [+1], from their previous series.

This series consists of a simple alphabetical order with the first two letters of all segments: B, C, D, E, F, G, H, I, J, K. The third letter of each segment is a repetition of the first letter.