Select the option that represents the letters which when placed sequentially from left to right in the blank spaces given in the series below will complete the series.
Q _ C _ Q S _ B _ S _ _ Q S _ _

The sequence has 16 positions. Let's try dividing it into equal blocks of 4 letters:
Q _ C _ | Q S _ B | _ S _ _ | Q S _ _

Let's find the repeating sequence by observing the common letters across blocks:

Block 2 begins with Q S. This implies the second letter in Block 1 is S.

Block 1 has C at the third position. This implies the third letter in Block 2 is C.

Combining these gives us the full pattern unit: Q S C B

Let's fill the blanks using this pattern unit:

First block: Q [S] C [B] → blanks: S, B

Second block: Q S [C] B → blank: C

Third block: [Q] S [C] [B] → blanks: Q, C, B

Fourth block: Q S [C] [B] → blanks: C, B

Gathering all filled blanks in sequential order: S B C Q C B C B.
This perfectly matches Option D.