Which of the following best explains why waterfowl feet are webbed?

Waterfowl have webbed feet primarily to facilitate swimming, which is crucial for their survival and feeding habits. The webbing acts as a paddle, allowing these birds to move efficiently through the water by providing a larger surface area against the water. This adaptation enables them to dive, forage for food underwater, and escape from predators more easily than if they had non-webbed feet.

In contrast, the other functions mentioned in the other options are not supported by the physical structure of webbed feet. Walking on land is less effective for waterfowl with webbed feet because it can hinder mobility on solid ground. Nesting purposes typically require stability and can be better served by feet with slightly different structures that allow for easier manipulation of materials. Lastly, perching on trees is not an activity associated with most waterfowl, as they are generally adapted for aquatic environments rather than arboreal habitats. Thus, the design of waterfowl's feet is primarily an evolution in response to their aquatic lifestyle, confirming that swimming is the best explanation.