Explanation:

Let's break down the letter-cluster series: qDx, nHs, kLn, hPi, ?

- Let's look at the first letter in each cluster:

* q ? n ? k ? h

* They decrease by 3 alphabet positions each time:

* q (17) ? n (14): -3

* n (14) ? k (11): -3

* k (11) ? h (8): -3

* So next: h (8) - 3 = e (5)

- Second letter in each cluster:

* D ? H ? L ? P

* Sequence increases by 4:

* D (4) ? H (8): +4

* H (8) ? L (12): +4

* L (12) ? P (16): +4

* So next: P (16) + 4 = T (20)

- Third letter:

* x ? s ? n ? i

* Descending by -5 each time:

* x (24) ? s (19): -5

* s (19) ? n (14): -5

* n (14) ? i (9): -5

* So next: i (9) - 5 = d (4)

Checking the options:

1. EtD

2. fTe

3. eTd

4. eSd

- Only option 3, eTd matches all the derived letters.

Correct Answer: Option 3 - eTd