Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.

EHF, JKM, ONT, ?

The correct option is TQA.

To find the missing letter-cluster in the given series, let us analyze the pattern followed by the letters in each position of the clusters: EHF, JKM, ONT, ?

1. First letter of each cluster:

E (5) → J (10) → O (15) → ?

The position value of each first letter increases by 5:

$5+5=10$ (J)

$10+5=15$ (O)

$15+5=20$ (T)

So, the first letter of the missing cluster is T.

2. Second letter of each cluster:

H (8) → K (11) → N (14) → ?

The position value of each second letter increases by 3:

$8+3=11$ (K)

$11+3=14$ (N)

$14+3=17$ (Q)

So, the second letter of the missing cluster is Q.

3. Third letter of each cluster:

F (6) → M (13) → T (20) → ?

The position value of each third letter increases by 7:

$6+7=13$ (M)

$13+7=20$ (T)

$20+7=27$

Since there are 26 letters in the alphabet, 27 corresponds to 27 - 26 = 1 (A).

So, the third letter of the missing cluster is A.

Combining all three letters, we get TQA.