The correct option is (C): bab
Explanation: To determine the missing letters in the sequence "ab—abb—bba—b," we need to analyze the pattern. The progression shows that the missing part should logically fit between the transitions from "ab" to "abb" to "bba" to "b." The letter combination "bab" fits this pattern, as it allows the sequence to flow smoothly. Therefore, the correct answer is C – "bab."