From the given options, select the group of letters that, when inserted in order from left to right into the blanks of the incomplete series, will logically complete it. S_ _KB_FH_BSF_K_SF_ _B

Correct — D.
Rule: the unit SFHKB repeats.

Unit 1: S F H K B
Unit 2: S F H K B
Unit 3: S F H K B
Unit 4: S F H K B

The filled letters, in order, are F H S K H B H K → option (d).

Why the others are wrong

• (a)
  FHSKHKHK — The sixth letter is K, but that blank closes unit 3, which must end in B. Unit 3 would read S F H K K.
• (b)
  FHSHKBHK — The fourth letter is H where unit 2 needs K, so unit 2 would read S F H H B, with no K in it.
• (c)
  FHSKBHHK — The fifth and sixth letters come as B, H, turning unit 3 into S F B K H instead of S F H K B.

This series is one short unit written out again and again. Find the unit from the letters you can see, then check that every blank falls into it.

The tail reads S F _ _ B and the start S _ _ K B. The 20 places split into four 5-letter units, and every given letter sits in the same slot of its unit: S first, F second, H third, K fourth, B fifth.

Key facts

• SFHKB SFHKB SFHKB SFHKB fills all 20 places.
• The blanks take F H S K H B H K.
• Split the series into equal units and check each blank against its slot.

Common traps

• Checking only the first few blanks: all four options begin F H S, so the difference sits further along

No directly related past PYQ was found.