Evolution is:
a) a change in the composition of a population from one generation to the next.
b) a phenomenon that must be inferred from evidence, because it cannot be observed directly.
c) the transformation of an individual to better fit its environment.
d) driven by the use or disuse of a body part, as when a finch uses its beak to crack open seeds.

A, b, and c are all correct.

take for example finches off of the galapagos islands. they vary from island to island, yet are still the same bird. on one island they have bigger, more durable and stronger beaks, because that island has many nuts to crack open and eat. on another island, they have thin ling beaks, ideal for pecking at insects in the grass on tree bark. evolution is literally survival of the ifttest, where those equipped best will live and those without means to fight for life will die off, leaving those most capable of reproduction and passing down genes.