Take another look at the sentence:
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.