Habit is to custom as transition is to volta. The correct option to fit the blank is VOLTA.
i will provide an explanation: In a sonnet, a volta refers to a turn of a thought or an argument. The Volta constitutes a significant part of a sonnet due to the fact it helps with transition, and it normally appears between the octave and the sestet of the sonnet - or if the sonnet is Shakespearean - before the final couplet.