In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
cd _ de _ dd _ ecdd _ e

Let's count the total number of spaces and letters: 15 slots.
Let's divide the pattern into groups of 5 letters each:
cd_de | _dd_e | cdd_e

Let's check Option C (dddc):

1st blank = d ⟹ cddde

2nd blank = d ⟹ dddde (doesn't quite form a repetitive cycle)

Let's rethink the grouping as a 5-letter repeating pattern: c d d d e

Group 1: c d [d] d e

Group 2: [c] d d [d] e

Group 3: c d d [d] e

Let's look at the filled combination: the blanks would be d, c, d, d. This matches the pattern cddde repeated 3 times.
The required letters are d c d d. This matches Option A.