Which of the following letters will sequentially replace the question marks (?) in the given series?
G, Z, I, X, K, V, ?, ?

The correct answer is M, T.

To find the missing letters in the series G, Z, I, X, K, V, ?, ?, let us analyze the pattern by splitting the series into two alternating sub-series.

First Sub-series (1st, 3rd, 5th, 7th letters):
G, I, K, ?

Let us look at their positions in the English alphabet:
• G = 7
• I = 9
• K = 11

The pattern here is adding 2 to the position of each previous letter:
$7+2=9$
$9+2=11$
$11+2=13$

The 13th letter of the alphabet is M.

Second Sub-series (2nd, 4th, 6th, 8th letters):
Z, X, V, ?

Let us look at their positions in the English alphabet:
• Z = 26
• X = 24
• V = 22

The pattern here is subtracting 2 from the position of each previous letter:
$26-2=24$
$24-2=22$
$22-2=20$

The 20th letter of the alphabet is T.

Therefore, the letters that replace the question marks are M, T.