What happens if the environment changes and a population in that environment has developed no new adaptations?

Two things can happen. First, the population moves out of the environment in search of a habitat suited for their needs. Or the population does not do anything, and according to Charles Darwin's theory of evolution, the strong survive, and the weak do not. In other words, the weaker, less adapted species in the environment die out, or go extinct. 

The correct answer is d