Explanation:

Here’s what’s really going on in this question:

- If you break down the clusters:

AOS, CQU, ?, GUY, IWA

-- Let’s put them in columns:

- First letters: A, C, ?, G, I

- Second letters: O, Q, ?, U, W

- Third letters: S, U, ?, Y, A

- Look for the pattern:

-- First letters: A (1), C (3), ?, G (7), I (9)

- Looks like odd numbers +2 each time: 1(A), 3(C), 5(E), 7(G), 9(I)

- So the missing letter is E.

-- Second letters: O (15), Q (17), ?, U (21), W (23)

- Jumps by +2: 15,17,19,21,23

- The missing letter is S (19).

-- Third letters: S (19), U (21), ?, Y (25), A (1 but cycles from 26)

- Jumps by +2 each time: 19,21,23,25,1

- The missing letter is W (23).

So the answer is ESW.

Option 1: ESW - Fits perfectly

Option 2: GSW - First letter doesn't match sequence

Option 3: FSX - None of the letters fit

Option 4: DRV - Letters off in all respects