Select the option that represents the letters that when placed from left to right in the blanks below will complete the letter series. _ Y Z K X K X _ Z K X _ X Y _ K X K X Y Z _ X K X Y

Correct — B. Cut the string into blocks of six and the pattern shows itself.

Rule: the six-letter block X Y Z K X K repeats, four times over, with X Y left dangling at the end.
_ Y Z K X K | X _ Z K X _ | X Y _ K X K | X Y Z _ X K | X Y

Gap 1 is slot 1 of block 1 → X
Gap 2 is slot 2 of block 2 → Y
Gap 3 is slot 6 of block 2 → K

Gap 4 is slot 3 of block 3 → Z
Gap 5 is slot 4 of block 4 → K

Read left to right the fills are X, Y, K, Z, K — option (b).

Why the others are wrong

• (a)
  KXYKZ — Option (a) opens with K. The first block already reads _ Y Z K X K, and only X completes X Y Z K X K, so the option is dead at gap 1 whatever the rest says.
• (c)
  XYZKK — Option (c) is right for the first two gaps and then puts Z at gap 3. Gap 3 closes block 2 — X _ Z K X _ — and every block here ends in K.
• (d)
  KYXZK — Option (d) breaks the block twice: K at gap 1, where the block starts with X, and X at gap 3, where the block ends with K. Its other three letters are right.

A repeating-block letter series is solved by finding the period before filling anything in. Guessing a blank from the letters on either side works only if you already know where the block starts.

Here the run K X K X after the first blank, and the X Y Z that keeps recurring, both point to a period of six. Once you have the period, each blank is answered by its position inside the block, and the neighbouring letters stop mattering.

Key facts

• The string holds 26 letters: four full runs of X Y Z K X K plus a trailing X Y.
• The five blanks fall at positions 1, 8, 12, 15 and 22 of the 26-letter string.
• The option has to supply all five fills in left-to-right order, so one wrong letter loses it.

Common traps

• Filling a gap from the letters either side instead of locating the repeating block first.
• Reading the options as words rather than as five separate fills in a fixed order.
• Assuming the string must finish on a complete block, and so mis-counting the period.

No directly related past PYQ was found.