Take a look at the sentence again:
Stuart wiggled his foot, drummed a Jimi Hendrix riff with his pencil, and
__________, but he could not burn off the caffeine-induced energy from an
extra large cup of coffee.
The blank needs to begin with a simple past tense verb. You chose was chewing a mouthful of gum, which is past progressive and thus causes a parallelism error. Before you continue, review the rules for parallel structure.
Go back to the sentence to try again.