A parasite is:AAn infection that returnsBA substance used for sterilizing surgical instrumentsCThe location of a diseased organDAn organism that lives off another organism

To determine the correct answer, let's analyze each option:

A. An infection that returns - This describes a recurring infection, not a parasite.

B. A substance used for sterilizing surgical instruments - This describes a sterilizing agent, not a parasite.

C. The location of a diseased organ - This describes a specific place in the body, not a parasite.

D. An organism that lives off another organism - This accurately describes a parasite, which relies on another organism (the host) for survival, often causing harm to the host.

Therefore, the correct answer is:

D. An organism that lives off another organism