ANSWER : babb

Explanation : . The pattern of the series is ccba/bbca/aabc/ccba/b. In each block of four letters, one letter is repeated twice.

ANSWER : ababba

Explanation : . The pattern of the series is baab/baab/baab/baab/b. A block of four letters baab is repeated.

ANSWER : acbb

Explanation : . A block of four letters bcaa is repeated. a/bcaa/bcaa/bcaa���.

ANSWER : cbab

Explanation : . The pattern of the series is aabc/abbc/abcc/aabc/abbc. In 1st block letter a is written twice, in 2nd block letter b is written twice and in 3rd block letter c is written twice.

ANSWER : bccbbb

Explanation : . A block of four letters ccbb is repeated. ccbb/ccbb/ccbb����..