Parasites are defined as:

• A. Organisms that live in or on another organism (host) and gets its nutrients from there.
• B. Organisms that cause illness.
• C. Microorganisms that require oxygen to survive.
• D. Non-living particles that may cause disease.

Answer: (A)

Parasites are defined by their relationship to a host: they live in or on another organism and obtain nutrients from that host. This dependence on the host for nourishment and survival is the key feature that sets parasites apart from other organisms. The option describing this host–nutrient relationship best captures what makes an organism a parasite, which is why it’s the correct choice.

Why the other ideas don’t fit: illness alone isn’t enough, because many organisms can cause disease without living on or in a host for nutrients. Requiring oxygen describes a metabolic trait, not a parasitic lifestyle. Non-living particles that may cause disease describe pathogens like prions or viruses in some contexts, but parasites are living organisms that rely on a host for sustenance, not inert particles.