Final answer:
Option (A), The eustachian tube's function is to equalize pressure between the nasopharynx and the middle ear, enabling proper eardrum vibration for hearing.
Step-by-step explanation:
The function of the eustachian tube is A) Pressure equalization. It is a small passage that connects the nasopharynx to the middle ear and allows for the balancing of air pressure on both sides of the tympanic membrane, which is essential for proper hearing. Without this pressure equalization, the eardrum cannot vibrate as effectively when it encounters sound waves.