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

The correct option is TXB.

To find the missing letter-cluster in the given series, let's analyze the pattern by looking at the positional values of the letters in the English alphabet (where A = 1, B = 2, ..., Z = 26):

First letter of each cluster:

D (4) → H (8) → L (12) → P (16)

Here, the first letter increases by 4 in each step:

16 + 4 = 20, which corresponds to the letter T.

Second letter of each cluster:

H (8) → L (12) → P (16) → T (20)

Similarly, the second letter increases by 4 in each step:

20 + 4 = 24, which corresponds to the letter X.

Third letter of each cluster:

L (12) → P (16) → T (20) → X (24)

The third letter also increases by 4 in each step (wrapping around the alphabet after Z = 26):

24 + 4 = 28 → (28 - 26 = 2), which corresponds to the letter B.

Combining the resulting letters gives the letter-cluster TXB.