Final answer:
The cuticle in free-living, terrestrial nematodes provides protection and support to the body. It is a flexible but tough exoskeleton made of chitin. This external skeleton offers physical protection and growth restriction.
Step-by-step explanation:
The selective advantage of a cuticle in free-living, terrestrial nematodes is that it provides protection and support to the body. The cuticle is a flexible but tough exoskeleton made of chitin, a carbohydrate-protein polymer. This external skeleton offers physical protection from predators, environmental conditions, and mechanical damage. Additionally, the cuticle restricts growth, meaning that nematodes must shed and replace it periodically as they increase in size.