Question

a pathogen is: a disease - causing microorganism a microorganism used in the production of food a type of parasite

Explanation:

A pathogen is defined as a disease - causing microorganism. It is not a microorganism used in food production (which would be like some bacteria in fermentation) and not just a general type of parasite (as pathogens can include bacteria, viruses, fungi etc., not all parasites are pathogens in the strict sense of causing disease in the way we commonly refer to pathogens).

Answer:

a disease - causing microorganism