Is there a body part that doesn’t have a lot of prosthetic options currently available? What market/users are currently underserved?

A body part that doesn't have a big number of prosthetic options is:

The eyes and other inner organs.

Explanation:

The reason behind this is that prosthetics have had a visual, and motor focus as a cause of their development. Meaning that they are the ones that have a major impact on the patient's lifestyle. Because they are at plain sight and are required to exercise movement. While eyes and inner organs are not that visually or motor impactful. Meaning that people won't know if you have a missing. Also, they are more difficult to be made. Because they have to perform very difficult tasks like filtering blood or any other substance.

the answer is a