Final answer:
d) Natural selection. Evolution, or changes in the genetic composition of biological populations over time, is directly observable through natural selection.
Step-by-step explanation:
Evolution, or changes in the genetic composition of biological populations over time, is directly observable. It is the result of natural selection, which is the process by which individuals with favorable traits have a greater chance of surviving and reproducing, passing their genes on to the next generation. For example, if a population of birds has individuals with longer beaks and individuals with shorter beaks, and there is a food source that requires longer beaks to access, over time the proportion of individuals with longer beaks will increase in the population.