If the environment changes, why can a previously useful trait become less useful later?

Correct answer: Because the advantage of a trait depends on environmental conditions

The governing concept is the environment-dependent nature of fitness. A trait is not universally beneficial; its value depends on factors such as climate, food, predators, disease, and competition. When these conditions change, a trait that once improved survival may provide little advantage or even become harmful. Selection may then favor different variants. Therefore A is correct; B and C deny environmental effects, while D wrongly claims that selection stops.