Break the whole string into equal blocks of 4 letters each and see if a repeating pattern emerges: 'ab_c', 'a_bc', 'ab_c', '_bbc'. If we guess the repeating unit is 'abbc', then: Block 1 (ab_c) needs 'b' in the blank to read 'abbc'. Block 2 (a_bc) needs 'b' in the blank to read 'abbc'. Block 3 (ab_c) again needs 'b'. Block 4 (_bbc) needs 'a' at the start to read 'abbc'. Reading the blanks in order gives b, b, b, a → 'bbba', which matches option (4).