What is the mechanism of natural selection?
a. any individuals in a population can be selected entirely by chance.
b. after a change in the environment a species will evolve adaptations to the new conditions.
c. if an adaptation to the environment is useful, an individual will develop it and pass it on to its offspring.
d. variations amongst individuals of a population are selected by a changing environment.

The answer is D. The natural selection is a theory which illustrates the gradual process of evolution in the population due to the change of environment. The key point is the mutation (variation) exists in the population before the environment changing. For C, if the environment does not change, though the adaptation is useful to environment, it will not be selected. Because the organisms without the adaptation will not die.