Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. B G L, G L Q, L Q V,Q V A, ?

Correct — C.
Rule: +5 at every position, read down the series.

First letters: B, G, L, Q → V(22).
Second letters: G, L, Q, V → V(22) + 5 = 27, past Z to A(1).
Third letters: L, Q, V, A → F(6).

That gives VAF → option (c).

Why the others are wrong

• (a)
  ZAS — ZAS has the right middle letter but starts with Z. The first letters climb B, G, L, Q in fives, so the next one is V, not Z.
• (b)
  OKS — OKS starts with O, which would send the first letters backwards from Q. They climb in fives, so the next first letter is V.
• (d)
  AVG — AVG swaps the first two letters: the series needs V first and A second. Its last letter is also wrong, since A + 5 is F, not G.

A letter-cluster series is three number series laid side by side. Convert letters to positions (A = 1 … Z = 26) and read one position at a time down the terms.

Here every position climbs by 5, and the letters inside each cluster are also 5 apart. So each cluster repeats the last two letters of the one before: BGL, GLQ, LQV, QVA. The answer must therefore begin VA.

Key facts

• A = 1 through Z = 26 is the position code these series run on.
• A step that runs past Z continues from A, so V + 5 = A and A + 5 = F.
• When the step between terms equals the gap inside each cluster, each cluster shares two letters with the next.

Common traps

• Stopping at V + 5 = 27 and looking for a 27th letter instead of wrapping to A
• Picking AVG because it holds A and V, without checking their order

No directly related past PYQ was found.