Which statement best defines parasitism?

• A. Neither organism benefits
• B. One organism benefits and the other is harmed
• C. One organism benefits, the other unaffected
• D. Both organisms benefit

Answer: (B)

Parasitism is a relationship in which one organism gains resources or benefits at the expense of another organism. The parasite relies on the host for things like nutrients, shelter, or transport, and the host is harmed in the process, which can range from mild to serious disease or reduced fitness. This concept is different from mutualism, where both organisms benefit (like pollinators and flowers), and from commensalism, where one benefits while the other is largely unaffected. So the statement that best defines parasitism is that one organism benefits and the other is harmed. For example, a tapeworm living in the intestines steals nutrients from the host, causing harm to the host.