Select the set of letters that, when sequentially placed in the blanks of the given letter series, will complete the letter series.
C R _ P _ _ T _ C _ T P

Let's count the total number of spaces including blanks: there are 12 positions.
Let's divide this into groups of 4:
C R _ P | _ _ T _ | C _ T P

Comparing the first group C R _ P and the third group C _ T P:
We can deduce that the pattern pattern could be C R T P.
Let's see if repeating C R T P fits all blocks:

First group: C R T P (Blank 1 = T)

Second group: C R T P (Blank 2 = C, Blank 3 = R, Blank 4 = P)

Third group: C R T P (Blank 5 = R)

Let's collect the filled letters in sequence: T, C, R, P, R.
This matches Option D perfectly.