Final answer:
A trait must be genetically controlled and heritable in order to be passed on to the next generation, which is the essence of natural selection. Evolution operates at the population level, not on individuals, and requires genetic changes that are heritable.
Step-by-step explanation:
The true statement about natural selection is: A) A trait must be genetically controlled and heritable to be passed on to the next generation. Natural selection explains how organisms in a population develop traits that allow them to survive and reproduce, meaning that traits offering an advantage will be more likely passed on to offspring; individuals with those traits have a better chance of surviving. This process does act on phenotypes, but for a trait to be passed on, it must affect the reproductive cells, so it can be inherited by the next generation. Populations evolve through genetics, not the individuals, making reproduction crucial for genetic changes to propagate within a population or species.
Individual adaptation or changes within an organism's lifetime, such as muscle development through exercise, are not examples of evolution, as these are not changes to the genetic makeup that can be passed down to the next generation. Evolution is the change in a population's genetic composition over time, reflected in the differential reproduction of individuals with certain alleles.