Which letter-cluster will replace the question mark (?) to complete the given series?
ETGW, ?, INOM, KKSH, MHWC

Let's break down the sequence by investigating the transformation of each letter position:
1st letter: E (5) → G (7) → I (9) → K (11) → M (13) [+2 steps]

2nd letter: T (20) → R (18) → N (14) → K (11) → H (8)

Let's check differences: 20−2=18 (R), 18−4=14 (N), 14−3=11 (K), 11−3=8 (H). This looks non-uniform. Let's re-examine standard option keys:

If pattern is constant −2 then: T(20) → R(18) → P(16). But next letter is INOM.

Let's re-verify positional values: T(20), R(18), N(14), K(11), H(8). The pattern of differences is −2,−4,−3,−3 which is unusual.

Let's check option GQKR: If 2nd letter is Q (17), then sequence is T(20) → Q(17) → N(14) → K(11) → H(8). This is a uniform sequence of −3 steps!

3rd letter: G (7) → K (11) → O (15) → S (19) → W (23) [+4 steps]

4th letter: W (23) → R (18) → M (13) → H (8) → C (3) [-5 steps]

Matching the verified patterns:

1st letter: 5+2=7→ G

2nd letter: 20−3=17→ Q

3rd letter: 7+4=11→ K

4th letter: 23−5=18→ R

Hence, the missing cluster is GQKR.