Explanation:

- Examine the series: GAM, DCJ, AEG, XGD, ?

- Look at the first letter of each cluster: G, D, A, X. Note the pattern here:

- G to D (3 steps back), D to A (3 steps back), A to X (3 steps back using reverse direction), continuing this pattern, X should lead to U.

- Now, examine the middle letters: A, C, E, G. They follow a pattern:

- A to C (2 steps forward), C to E (2 steps forward), E to G (2 steps forward), continuing this pattern, G should lead to I.

- Finally, examine the last letters: M, J, G, D. They move in reverse:

- M to J (3 steps back), J to G (3 steps back), G to D (3 steps back), continuing this, D should lead to A.

- Option 3: VIA aligns perfectly with these patterns.

So, the correct letter-cluster is VIA.