Why Is 'Genetic Variation' in a Population's Gene Pool Helpful

Why is 'genetic variation' in a population's gene pool helpful for species survival?

A) If only a small portion of a population varies genetically, then only a small portion dies.
B) It provides for broader variety during the process of mate selection.
C) It is not helpful; it makes extinction more likely.
D) Genetic variation means that some members of a population faced with an environmental pressure will have a better chance of surviving it.
E) Where there is no variation, every organism can survive an environmental change.