Shortly before she was to start high school, aaliyah's family moved to another state. on the first day of school, her mother catches her sucking her thumb, something that aaliyah hasn't done in years. which defense mechanism is aaliyah using to avoid stress?

Regression.

Regression is a defense mechanism leading to reversion of the ego to an earlier stage of psychological development  instead of handling  in a more adult way.
In Aaliyah's case, she got stuck in the oral stage. This stage goes from birth to eighteen months of age. The main focus is pleasure seeking through the infant’s mouth, they need to taste and suck. if not,  the infant will be fixated leading to adult habits such as smoking, thumb-sucking, over-eating, and baiting nails.