The correct answer is C. Mistletoe growing on an orange tree.

Mistletoe is a parasitic plant that grows on the branches of another plant (in this case, an orange tree). It derives its nutrients from the host plant, harming it in the process.

The other options are not examples of parasitism:

A. Fungi growing on a dead tree branch is an example of saprophytism (obtaining nutrients from dead organic matter).

B. A squirrel living in an abandoned nest is an example of commensalism (one organism benefits, the other is not affected).

D. Cattle egrets taking ticks from the body of cattle is an example of mutualism (both organisms benefit).