Select the combination of letters that when sequentially placed from left to right in the blanks of the given series will complete the letter series.
A _ _ O U A E I _ _ A _ I _ U A E _ O _

The series is based on a repeating sequence of the five English vowels: A, E, I, O, U.
Let's see if the continuous block repeats as "A E I O U":

First group: A E I O U (Blanks 1 and 2 filled with E, I)

Second group: A E I O U (Blanks 3 and 4 filled with O, U)

Third group: A E I O U (Blanks 5 and 6 filled with E, O)

Fourth group: A E I O U (Blanks 7 and 8 filled with I, U)

Putting it all together, the full series becomes:
A E I O U A E I O U A E I O U A E I O U

The sequence of letters filled into the blanks is: E, I, O, U, E, O, I, U.
This matches option C: EIUOEOIU.