Final answer:
A genetic predisposition to fear darkness that contributes to reproductive success exemplifies natural selection, as it describes the increase in frequency of a heritable trait that provides a survival or reproductive advantage.
Step-by-step explanation:
If a genetic predisposition to fear darkness contributes to reproductive success, the trait making individuals more likely to survive and reproduce would be passed on to subsequent generations. This scenario best illustrates the concept of natural selection. Natural selection acts on heritable traits that are advantageous for survival and reproduction, thereby increasing the frequency of those traits in the population.
The field of study that focuses on genes and their expression is epigenetics, which can influence how genes are turned on or off, but the direct passage of fear of darkness as a successful trait aligns more closely with natural selection. Kin selection is another aspect of evolutionary theory, where behaviors are favored that help close relatives, who share some of the individual's genes, to survive and reproduce. Behaviors that are genetically determined and contribute to an individual's reproductive success, including fight or flight responses, are subject to natural selection and may become more prevalent in the population over time.
Thus, the most accurate answer to the question regarding the genetic predisposition to fear darkness is d. natural selection.