Which letter-cluster will complete the given series?
DHMX, GJKU, _________ , MNGO

The correct option is JLIR.

To find the missing letter-cluster, let's analyze the pattern followed by each letter position across the given series:

Given series: DHMX, GJKU, _________ , MNGO

1. First letter of each cluster:
D (4) + 3 = G (7)
G (7) + 3 = J (10)
J (10) + 3 = M (13)

2. Second letter of each cluster:
H (8) + 2 = J (10)
J (10) + 2 = L (12)
L (12) + 2 = N (14)

3. Third letter of each cluster:
M (13) - 2 = K (11)
K (11) - 2 = I (9)
I (9) - 2 = G (7)

4. Fourth letter of each cluster:
X (24) - 3 = U (21)
U (21) - 3 = R (18)
R (18) - 3 = O (15)

Combining the missing letters from each position yields JLIR.