Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
_ P G _ _ M P _ _ A _ _ G F A

Let's find the total number of letters including spaces.
The given sequence is: _ P G _ _ M P _ _ A _ _ G F A
Total elements = 15.

Let's try breaking it down into sets of 5 elements:

_ P G _ _

M P _ _ A

_ _ G F A

By comparing the parts:

From set 2, the first character is 'M'. This implies the first blank in set 1 is likely 'M' → M P G _ _

From set 3, the last two letters are 'F A'. This suggests the last two positions of sets 1 and 2 are 'F A'.

Let's check: set 1 becomes M P G F A. Set 2 becomes M P G F A. Set 3 becomes M P G F A.

This confirms the repeating 5-letter block pattern is M P G F A.

Let's list the filled letters in sequence:

1st blank: M

2nd blank: F

3rd blank: A

4th blank: G

5th blank: F

6th blank: M

7th blank: P

The sequence of letters inserted is: MFAGFMP.