Correct Answer: a c a c a b

Solution : Given -

a _ b _ ca _ b _ c _ a _ cc

Let's check the options.

option first: a c b c a b

The series becomes a a b c ca b b c c a a b cc

There is no pattern in the above series.

option second: a b a b c a

The series becomes a a b b ca a b b c c a a cc

There is no pattern in the above series.

option third: a c a c a b

The series becomes a a b c ca a b c c a a b cc

The pattern in the series becomes aabcc/aabcc/aabcc.

option fourth: a b a b a c

The series becomes a a b b ca a b b c a a c cc

There is no pattern in the above series.

Hence, third option is correct.