Which one set of letters, when sequentially placed in the blanks, will…

Place the letters of option B sequentially in the five blanks: [a] bbca [c] bcca [b] ac [a] a [b] cb. This gives the grouped series: abbc / ac / bcca / ba / caab / cb. The four-letter groups rotate cyclically: abbc -> bcca -> caab. After each four-letter group, the two-letter group uses that group's first and last letters: ac, ba, cb. Hence the required letters are a, c, b, a, b, i.e. acbab.