Explanation:

Alright, let’s break down this letter-cluster series: ZOFB, ?, VKJF, TILH, RGNJ.

First, notice the letters move backwards in the alphabet in pairs:

- 1st letter: Z, ?, V, T, R (Z?V is -4, V?T is -2, T?R is -2; but if you check stepwise: Z?? (how many steps?), so pattern seems -2 after -4)

- 2nd letter: O, ?, K, I, G (Same jump: O?K is -4, K?I is -2, etc.).

- 3rd letter: F, ?, J, L, N (F?J is +4, J?L +2, L?N +2; so each step goes up).

- 4th letter: B, ?, F, H, J (B?F is +4, F?H +2, H?J +2).

So, for the missing spot:

- 1st letter: Z (26) minus 2 = X (24).

- 2nd letter: O (15) minus 2 = M (13).

- 3rd letter: F (6) plus 2 = H (8).

- 4th letter: B (2) plus 2 = D (4).

That spells out XMHD.

- Let's check options:

- Option 1: XNIF (doesn’t fit)

- Option 2: BQJF (nope)

- Option 3: XMHD (matches our work)

- Option 4: BNHE (no).

Bottom line:

The missing cluster is XMHD.