Find the correct combination of letters that when sequentially filled in the blanks of the given series will make the series logically complete. ABF_HI_NOP_U_WABCDH_

Correct — C. Five blanks in a twenty-letter string means blocks of four. Cut the string there and the pattern shows.

Rule: inside a block the steps are +1, +4, +1, and each block starts seven letters after the one before.

ABF_ becomes A, B, F, G — A+1=B, B+4=F, F+1=G.
HI_N becomes H, I, M, N.
OP_U becomes O, P, T, U.

_WAB becomes V, W, A, B, with W+4 wrapping to A.
CDH_ becomes C, D, H, I.

The block openings then read A, H, O, V, C — +7 each time, with V+7 wrapping to C. The blanks in order are GMTVI → option (c).

Why the others are wrong

• (a)
  GMSVI — GMSVI puts S in the third block, which reads O, P, ?, U. The step from P is +4, and that lands on T.
• (b)
  GMTUI — GMTUI puts U where the fourth block opens. That block reads ?, W, A, B, and the opening step is +1, so the letter before W is V.
• (d)
  IMTVI — IMTVI starts the string A, B, F, I. F + 1 is G, and I would also break the +7 spacing between block openings.

A long unbroken string of letters is usually blocks laid end to end. Finding the block length is the whole problem, and the count of blanks is the strongest hint you get.

Five blanks and twenty letters points straight at blocks of four with one blank each. Once cut, each block is short enough to read by eye, and the openings form a series of their own.

Key facts

• Each four-letter block steps +1, then +4, then +1.
• The block openings run A, H, O, V, C, seven letters apart each time.
• The completed string is ABFG HIMN OPTU VWAB CDHI.

Common traps

• Reading the string as one long series instead of blocks laid end to end.
• Missing the wraps at W + 4 and V + 7.
• Filling only the blanks you can see without checking the block openings.

No directly related past PYQ was found.