Which set of letters, filled into the blanks in order, completes the repeating pattern a _ c a _ c a b _ a b _ ?

Explanation

1. 01

   Look for a block that repeats. The visible letters fit the block abc: a_c | a_c | ab_ | ab_.

2. 02

   Fill each block as abc: the blanks are b, b, c, c.

3. 03

   The complete pattern is abcabcabcabc.