The answer is A. Ear.
The other three options, B, C, and D, are all organs that are essential for life. The ear is not an essential organ, and it is possible to live without one.
The ear is a sensory organ that allows us to hear. It is made up of three parts: the outer ear, the middle ear, and the inner ear. The outer ear is the part of the ear that we can see. It is made up of the ear canal and the eardrum. The middle ear is a small, air-filled space behind the eardrum. It contains three small bones called the ossicles. The ossicles transmit sound waves from the eardrum to the inner ear. The inner ear is a fluid-filled space that contains the cochlea and the vestibular system. The cochlea is responsible for hearing. The vestibular system is responsible for balance.
The ear is a very important organ, but it is not essential for life. It is possible to live without an ear, and there are many people who do. People who are born without an ear or who have lost an ear can still live normal lives. They may need to wear a hearing aid or use other methods to communicate, but they can still do everything that other people can do.
The other three options, B, C, and D, are all organs that are essential for life. The kidney is an organ that filters waste products from the blood. The lungs are organs that take in oxygen and release carbon dioxide. The liver is an organ that helps to digest food and detoxify the body. All of these organs are necessary for the body to function properly.