Janice is known for being extremely __________; yet, it's challenging to envision her as a professional comedian.

The sentence suggests a contrast between Janice's known characteristic and the expectation of her performance as a professional comedian. Being "extremely witty" aligns perfectly with someone who would typically succeed in a comedy setting, as wit involves quick thinking and the ability to make clever, humorous remarks. This wit could translate well into comedic routines, making it conceivable for her to perform as a comedian.

In this context, the other options do not contribute to the idea of her potentially succeeding as a comedian. While "dull" would indicate a lack of humor, and "garrulous" refers to being excessively talkative without necessarily implying wit, "effusive" describes someone who expresses feelings of gratitude, pleasure, or approval in an unrestrained manner, which may not align with the sharp humor expected of a comedian. Thus, "witty" emerges as the appropriate choice to contrast with the challenge of envisioning her as a successful professional in comedy.