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

Let the total number of letters including blanks be counted.
The given series is: S A _ C _ _ S _ D _ A T _ _ D C A _ S _ D C _ T
Total characters = 24.
We can divide the series into 4 equal groups of 6 letters each:
S A _ C _ _ | S _ D _ A T | _ _ D C A _ | S _ D C _ T

By comparing the blocks with one another:

The general pattern of each group seems to be S A D C A T.

Let's test this repeating sequence (S A D C A T):
1st group: S A D C A T -> S A D C A T
2nd group: S A D C A T -> S A D C A T
3rd group: S A D C A T -> S A D C A T
4th group: S A D C A T -> S A D C A T

The missing letters picked sequentially from left to right are:
D, A, T, A, C, S, A, T, A, A

This matches option A.