Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
CO_ _ G_C _ E_E_OL _GE

Let's count the total number of characters (including blanks) in the given series:
CO_ _ _G_C_ _ _E_E_OL_ _GE has 21 positions.

Let's check if there is a repeating pattern of 7 letters (7×3=21):
C O G L E O G | C O G L E O G | C O G L E O G

Let's fit Option D (L, G, L, E, O, C, O, G, L, G, E) into the blanks step-by-step:

CO + L + G + L + G + E → COLGLGE

C + O + C + O + E + G + E → COCOEGE (This does not create a uniform pattern).

Let's re-examine the word form. A very common meaningful word pattern here is COGEL or COGLE or COLLEGE.
Let's check the length of COLLEGE: It has 7 letters.
Let's test if the repeating unit is C O L L E G E:

1st block: C O L L E G E → Blanks: L, L, E

2nd block: C O L L E G E → Blanks: O, L, L

3rd block: C O L L E G E → Blanks: G, C, L, E

Let's combine these blanks in sequence: L, L, E, O, L, L, G, C, L, E.
This matches Option A perfectly.

Let's verify by substituting Option A into the main text:
CO + L + L + E + G + O + C + L + L + E + G + E + OL + C + L + GE
This correctly forms: COLLEGE | COLLEGE | COLLEGE.