Which of the following diseases is caused by parasites?

The correct response is that parasitic diseases are specifically caused by parasites, which are organisms that live on or inside a host, deriving benefits at the host's expense. These diseases can result from various types of parasites, including protozoa, helminths, and ectoparasites such as fleas and lice. Examples of parasitic diseases include malaria, caused by the Plasmodium parasite, and giardiasis, caused by Giardia lamblia.

The other choices do not pertain to diseases caused by parasites. Cancer, for instance, is a group of diseases characterized by uncontrolled cell growth and is not linked to parasitic infections. Diabetes is a chronic condition resulting from issues with insulin production or usage in the body and is not related to any parasitic organisms. Pneumonia, while it can be caused by various pathogens, is primarily a respiratory infection predominantly caused by bacteria and viruses rather than parasites. Thus, recognizing that parasitic diseases are those specifically tied to parasitic infections is key to understanding this topic.