Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series. B_DE_GDGBK_IB_DK

Correct — D. Sixteen letters and four blanks: split the string into blocks of four.

B _ D E | _ G D G | B K _ I | B _ D K

Rule: every block reads B ? D ? — first letter B, third letter D. That alone fills the second blank with B and the third blank with D.

The second letters of the blocks run C, G, K, O, rising by 4 each time, so the first blank is C and the fourth is O. The fourth letters, E, G, I and K, rise by 2 and are all given already.

Blanks in the order they appear: C, B, D, O — option (d).

Why the others are wrong

• (a)
  CBDP — The first three letters are right, but the last block would then read B P D K. The second letters rise by 4, and K + 4 is O, not P.
• (b)
  OCBD — The right four letters in the wrong order. Blanks are filled left to right, and the first blank is the second letter of block one, which is C.
• (c)
  CODB — Puts O in the second blank, but that blank is the opening letter of its block, and every block starts with B.

A blank-filling letter series is solved by finding the block length first, never by reading letter to letter.

Sixteen positions and four blanks point at blocks of four, and the given letters confirm it: B stands at positions 1, 9 and 13, and D at positions 3, 7 and 15. Lock those two columns down before touching anything else.

Then read each column of the blocks as its own progression. Column two gives C, G, K, O, a rise of 4. Column four gives E, G, I, K, a rise of 2.

Key facts

• The series is four blocks of the form B ? D ?, so positions 1 and 3 of every block are fixed.
• Column two of the blocks runs C, G, K, O, a rise of 4 in the alphabet.
• Column four runs E, G, I, K, a rise of 2.
• An option must list the blanks in the order they appear from left to right.

Common traps

• Reading the string as one long progression instead of splitting it into blocks
• Finding the right four letters and then writing them in the wrong order
• Counting the alphabet gap inclusively, which turns a rise of 4 into a rise of 5

No directly related past PYQ was found.