Final answer:
The Eustachian tube is responsible for equalizing air pressure in the middle ear.
Step-by-step explanation:
The structure responsible for equalizing air pressure in the middle ear is the Eustachian tube. The Eustachian tube connects the middle ear to the pharynx and allows air to enter or leave the middle ear, equalizing pressure on both sides of the eardrum. When there is a pressure difference between the middle ear and external environment, the Eustachian tube opens to equalize the pressure, preventing discomfort or damage to the eardrum.