Select the letter-cluster from among the given options that can replace the question mark (?) in the following series:
CDEF, CKSA, CRGV, CYUQ, ?

The correct answer is Option 4: CFIL.

To find the missing letter-cluster that replaces the question mark (?), let us analyze the pattern followed by each letter position across the given series:

CDEF, CKSA, CRGV, CYUQ, ?

Let us determine the positional values of the letters in the English alphabet:
A = 1, B = 2, C = 3, D = 4, E = 5, F = 6, G = 7, H = 8, I = 9, J = 10, K = 11, L = 12, M = 13, N = 14, O = 15, P = 16, Q = 17, R = 18, S = 19, T = 20, U = 21, V = 22, W = 23, X = 24, Y = 25, Z = 26.

First letter of each cluster:
C (3) ⇒ C (3) ⇒ C (3) ⇒ C (3) ⇒ ?
The first letter remains constant as C.

Second letter of each cluster:
D (4) + 7 = K (11)
K (11) + 7 = R (18)
R (18) + 7 = Y (25)
Y (25) + 7 = 32 - 26 = 6, which corresponds to F.

Third letter of each cluster:
E (5) + 14 = S (19)
S (19) + 14 = 33 - 26 = G (7)
G (7) + 14 = U (21)
U (21) + 14 = 35 - 26 = 9, which corresponds to I.

Fourth letter of each cluster:
F (6) - 5 = A (1)
A (1) - 5 = 27 - 5 = V (22)
V (22) - 5 = Q (17)
Q (17) - 5 = 12, which corresponds to L.

Combining all the letters obtained for each position, the required cluster is CFIL.