Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.AGMZ, CEOX, ECQV, ?

Let's look at the pattern for each individual letter in the clusters:First letter: A $\to$ C $\to$ E $\to$ ?A(1) $+2=$ C(3)C(3) $+2=$ E(5)E(5) $+2=$ G(7)Second letter: G $\to$ E $\to$ C $\to$ ?G(7) $-2=$ E(5)E(5) $-2=$ C(3)C(3) $-2=$ A(1)Third letter: M $\to$ O $\to$ Q $\to$ ?M(13) $+2=$ O(15)O(15) $+2=$ Q(17)Q(17) $+2=$ S(19)Fourth letter: Z $\to$ X $\to$ V $\to$ ?Z(26) $-2=$ X(24)X(24) $-2=$ V(22)V(22) $-2=$ T(20)Combining these letters gives the cluster GAST.