A parasite is:AAn infection that returnsBThe location of a diseased organCAn organism that lives off another organismDA substance used for sterilizing surgical instruments

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

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

B. The location of a diseased organ - This refers to the place where an organ is affected by disease, not a parasite.

C. An organism that lives off another organism - This accurately describes a parasite, which is an organism that lives on or in a host organism and gets its food at the expense of its host.

D. A substance used for sterilizing surgical instruments - This refers to a sterilizing agent, not a parasite.

Therefore, the correct answer is:

C. An organism that lives off another organism