Select the option that represents the letters that, when sequentially placed from left to right in the blanks below, will complete the letter series.
C _ C _ S _ C _ _ C _ _

The given series has 12 positions: C _ C _ S _ C _ _ C _ _
Let's test the options to find a repeating pattern:
If we use option D (SCCSCSC), the sequential letters to fill the blanks are:
1st blank = S
2nd blank = C
3rd blank = C
4th blank = S
5th blank = C
6th blank = S
7th blank = C

Placing these into the series:
C S C C S C C S C C S C

Breaking this complete sequence into blocks of 3 letters:
CSC | CCS | CSC | CSC

We can observe a highly regular repeating pattern where CSC alternates or repeats dominantly:
Block 1: CSC
Block 2: CCS (or CSC if a minor variation, but option D provides a beautifully structured rhythmic sequence)

Let's re-verify by breaking into blocks of 4:
CSCC | SCSC | CSCC -> This reveals a strict repeating pattern of the 4-letter block CSCC.

Block 1: C S C C

Block 2: S C C S

Block 3: C C S C

Let us re-examine the core block: C C S or C S C.
If we divide the 12-letter sequence into 4 groups of 3 letters:

C S C

C S C

C S C

C S C

Let's check which option gives exactly CSC CSC CSC CSC:
The original string with blanks: C [1] C [2] S [3] C [4] [5] C [6] [7]
To make it CSC CSC CSC CSC:

Position 2 (1st blank) = S

Position 4 (2nd blank) = C

Position 6 (3rd blank) = C

Position 8 (4th blank) = S

Position 9 (5th blank) = C

Position 11 (6th blank) = S

Position 12 (7th blank) = C

The sequence of letters filled is: S, C, C, S, C, S, C.
This matches Option D perfectly.