Select the option that represents the letters that, when sequentially placed from left to right in the following blanks, will complete the letter-series.
R _ G _ D _ T G _ D R _ G F _ R T _ _ D

Let's split the 20-letter series into 4 blocks of 5 letters each:
R _ G _ D | _ T G _ D | R _ G F _ | R T _ _ D

By comparing columns and letters across blocks, we can see a clear repeating pattern of R T G F D:

Block 1: R T G F D

Block 2: R T G F D

Block 3: R T G F D

Block 4: R T G F D

Collecting the blank values in order gives: T, F, R, F, T, D, G, F.
This corresponds to option C.