Explanation:

Let’s break this down step by step:

- Look at the pattern between the clusters: bisq, dguo, ?, hcyk.

- Compare each letter’s position in the alphabet.

- b (2) ? d (4): +2; d (4) ? h (8): +4.

- i (9) ? g (7): -2; g (7) ? c (3): -4.

- s (19) ? u (21): +2; u (21) ? y (25): +4.

- q (17) ? o (15): -2; o (15) ? k (11): -4.

- The jumps are: +2, -2, +2, -2, then +4, -4, +4, -4.

So for the missing cluster:

- Use: +2 on d (4): f (6)

- Use: -2 on g (7): e (5)

- Use: +2 on u (21): w (23)

- Use: -2 on o (15): m (13)

So the answer is fewm.

fewm is the correct option.

As for the other options:

- eevm and efvn don’t follow the sequence of letter shifts.

- ffwn also doesn’t maintain the alternating plus and minus pattern.

So, “fewm” is the only one matching the series.