Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
G _ I N G G O _ N _ G _ I N _ G O I _ _

Let's count the total number of characters including the blanks:
The length of the string is 21 letters. Let's look for repetitive blocks or patterns like 'GOING'.
Let's see if the word GOING is repeated:
G O I N G | G O I N G | G O I N G | G O I N G

Let's check the letters inserted in the blanks to complete this pattern:

1st blank: O

2nd blank: I

3rd blank: G

4th blank: O

5th blank: G

6th blank: N

7th blank: G

Combining these filled letters in order yields: OIGOGNG.
This perfectly matches Option C.