Take a look at the sentence again:
Natalya quietly unzipped her book bag, removed a bottle of pink polish,
and __________ while Dr. Adams droned on and on about the Mesopotamians.
The blank needs to begin with a simple past tense verb. You chose she began painting her fingernails, which is a main clause and thus causes a parallelism error. Before you continue, review the rules for parallel structure.
Go back to the sentence to try again.