What function does the eustachian tube serve?

The eustachian tube is primarily responsible for equalizing pressure between the middle ear and the external environment. This function is crucial for maintaining proper hearing and preventing discomfort that can arise from pressure changes, such as during plane travel or diving. When the eustachian tube opens, it allows air to flow into the middle ear, thus balancing the pressure on either side of the eardrum.

Understanding the other options helps clarify the role of the eustachian tube: While it indeed connects the throat to the middle ear, that connection alone does not define its primary function. Transmitting sound is a role of the structures in the ear, particularly the ossicles, and not related to the eustachian tube. Finally, balance is mainly maintained by the inner ear structures such as the vestibular system, rather than the eustachian tube, which focuses on pressure regulation. Thus, option B is key to understanding the physiological importance of the eustachian tube in auditory function and ear health.