Explanation:

- The given series is: bisq, dguo, ?, hcyk.

- We need to identify the pattern in the series.

- Analyze the positions of each letter in the alphabet.

- 'b' (2), 'd' (4), 'h' (8) shows an arithmetic progression in steps of +2.

- 'i' (9), 'g' (7), 'c' (3) shows an arithmetic sequence in steps of -2.

- 's' (19), 'u' (21), 'y' (25) shows steps of +2.

- 'q' (17), 'o' (15), 'k' (11) shows steps of -2.

- Following this pattern, the missing cluster should be efvn.

- Option 3: efvn is the correct answer.

efvn should replace the question mark in the series.

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