Final answer:
The tympanic membrane optimally vibrates when the air pressure in the middle ear equals the external auditory canal pressure, allowing for proper sound wave transmission and hearing.
Step-by-step explanation:
The tympanic membrane vibrates optimally when the air pressure in the middle ear cavity equals the air pressure in the external auditory canal (D). This is necessary for efficient transmission of sound waves. When sound waves travel through the auditory canal, they cause the tympanic membrane, also known as the eardrum, to vibrate. This vibration is then transferred across the middle ear ossicles - the malleus, incus, and stapes - and is amplified before being transmitted to the cochlea via the oval window. In the cochlea, the movement of the stapes against the oval window generates pressure waves in the fluid, leading to the stimulation of the hair cells and ultimately the transmission of electrical signals to the brain.