How Is It Possible for a Mutation to Be Beneficial

How is it possible for a mutation to be beneficial and what happens to it if it is?

A) A beneficial mutation will result in a trait that is helpful to the organism and if the environment changes to suit it, the trait will expand in the population.
B) A mutation is, by definition, not beneficial.
C) A mutation can result in a trait that is beneficial to an individual or its offspring and it will stay within that family.
D) It is not possible for a mutation to be beneficial and so the mutated gene will be selected against and disappear from the population.
E) A mutation that results in a trait that improves an organisms fitness is beneficial and because it has improved fitness that mutated gene will spread in the population.