Which statement about symbiosis is true?

• A. Mutualism is when both species benefit; commensalism is when one benefits and the other is unaffected.
• B. Commensalism is when both species benefit.
• C. Parasitism is when neither benefits.
• D. Mutualism occurs when both species benefit, such as gut microbes aiding digestion in mammals.

Answer: (D)

Mutual benefit is the defining feature here: in a mutualistic relationship, both species gain from the interaction. The statement captures this by saying mutualism occurs when both partners benefit and by giving a concrete example—the gut microbiome helping mammals digest food. In the gut, microbes break down plant fibers and synthesize vitamins, while the host provides a steady habitat and nutrients. This reciprocal advantage is what makes mutualism distinct.

For context, other forms of symbiosis exist: one partner may benefit while the other is unaffected (commensalism), or one partner may benefit at the expense of the other (parasitism). But the combination of a clear mutual benefit and a real-world example makes this statement a strong, concrete depiction of mutualism.