Which of the following defines parasites?

Parasites are defined as organisms that live on or inside a host and obtain nutrients at the host's expense, often harming the host in the process. Toxoplasmosis, which is caused by the protozoan parasite Toxoplasma gondii, exemplifies a typical parasite. It can manipulate the behavior of its host and may lead to various health issues, particularly in immunocompromised individuals or pregnant women.

The other options present definitions or characteristics that do not accurately describe parasites. Rapid cell death is not a defining feature of parasites, as many pathogens can cause such effects without being classified as parasites. The description of microorganisms that are always beneficial does not relate to parasites since parasites are generally harmful rather than beneficial. Lastly, fungi that live in extreme environments speaks to a specific ecological niche rather than the nature of parasitism. Therefore, option C is the appropriate choice as it clearly aligns with the established definition of parasites.