Final answer:
Adaptation is a heritable trait that helps an organism survive and reproduce in its environment. It arises through evolution by natural selection, where traits that increase fitness become more common in a population over time.
Step-by-step explanation:
An adaptation is best defined as any heritable trait or behavior in an organism that aids in its survival and reproduction in its present environment. This means that through the process of evolution by natural selection, certain genetic variations become favored because they provide an advantage to the organism in a specific environment. For example, the diversity in the beak sizes of Darwin's finches is an adaptation to the varying availability of seeds, allowing them to better survive and reproduce based on food resources. In essence, an adaptation increases the 'fit' of the population to its environment.
Given the options provided, the best definition for an adaptation is C. any trait that increases fitness.