Here is the passage again:
Professor Gregory surveyed the auditorium full of nervous, sweating biochemistry
students. Her gaze often stopped on Bryan. Whose test
anxiety manifested in the loud popping of his knuckles and the high-speed
bouncing of his left leg.
The third part is a subordinate clause fragment. Although it has a subject, anxiety, and a verb, manifested, the relative pronoun whose keeps it from making a complete thought.