Select the letter-cluster that can replace the question mark (?) in the given series.
LMP , NOR, PQT, ?

The correct option is RSV.

To find the missing letter-cluster, let us analyze the pattern followed by the letters in each position across the series:

1. First Letter of Each Cluster:

L → N → P → ?

Looking at their alphabetical positions:

L (12) + 2 = N (14)

N (14) + 2 = P (16)

P (16) + 2 = R (18)

2. Second Letter of Each Cluster:

M → O → Q → ?

Looking at their alphabetical positions:

M (13) + 2 = O (15)

O (15) + 2 = Q (17)

Q (17) + 2 = S (19)

3. Third Letter of Each Cluster:

P → R → T → ?

Looking at their alphabetical positions:

P (16) + 2 = R (18)

R (18) + 2 = T (20)

T (20) + 2 = V (22)

Combining the resulting letters from each position gives the letter-cluster RSV.