The correct answer is option 'A', which is air. Let's understand why air is present in the cavity of the middle ear in humans.

Air-filled Cavity:
The middle ear is a small, air-filled cavity located behind the eardrum, also known as the tympanic membrane. It is situated between the outer ear and the inner ear. The middle ear is an important part of the auditory system as it plays a key role in transmitting sound vibrations from the outer ear to the inner ear.

Function of Air:
The presence of air in the middle ear serves several important functions:

1. Sound Transmission: Air is an excellent medium for transmitting sound waves. When sound enters the ear canal, it causes the eardrum to vibrate. These vibrations are then transmitted through the air-filled middle ear to the inner ear, where they are converted into electrical signals and interpreted by the brain as sound.

2. Pressure Equalization: The middle ear is connected to the back of the throat by a narrow tube called the Eustachian tube. This tube helps equalize the pressure between the middle ear and the outside environment. When the pressure in the middle ear becomes unequal, such as during changes in altitude (e.g., flying in an airplane), the Eustachian tube opens to allow air to flow in or out, equalizing the pressure and preventing discomfort or damage to the eardrum.

3. Protection: The air-filled middle ear acts as a cushion to protect the delicate structures of the inner ear, such as the cochlea. The cochlea is responsible for converting sound vibrations into electrical signals that can be interpreted by the brain. The air-filled cavity helps to absorb and dissipate any excessive pressure or vibrations, preventing damage to the inner ear.

Conclusion:
In conclusion, the presence of air in the cavity of the middle ear in humans is essential for the transmission of sound vibrations, pressure equalization, and protection of the inner ear. It provides an optimal medium for sound transmission and helps maintain the health and functioning of the auditory system.