Natural selection causes changes in populations of organisms, not individual organisms, because a. differential reproductive success caused by acquired behaviors is the most important mechanism of natural selection. b. differential reproductive success caused by genetic variation is necessary for the process of natural selection. c. organisms within sexually reproducing populations can only pass on genes that reduce their reproductive success. d. organisms within sexually reproducing populations all have the same genetic information and the same likelihood of survival.

B. differential reproductive success caused by genetic variation is necessary for the process of natural selection.

Explanation:

Natural selection is the process wherein individuals with adaptive genetic variations are favored. Genetic variations are present among the individuals of a population. Individuals with adaptive genetic variations survive more and reproduce to leave more progeny. Their progeny also carries the selected adaptive genetic variations.

Over the generations, the population evolves into a population with more number of individuals carrying those beneficial genetic variations.

Since natural selection favors the individuals with the genetic traits that make them better adapted to prevailing surroundings, it leads to the evolution of populations.  

"a.) well–developed vascular system."