Final answer:
Natural selection requires variation in fitness and variation in traits within a population for it to operate. The correct answer is either A or B.
Step-by-step explanation:
Natural selection requires variation in one or more traits of individuals within a population in order to operate. This means that individuals in the population must have different characteristics or traits that are inheritable. For example, in a population of birds, some individuals may have longer beaks while others have shorter beaks.
Additionally, natural selection also requires variation in fitness among individuals within a population. Fitness refers to the ability of an individual to survive and reproduce. Individuals with traits that give them an advantage in their environment are more likely to survive and produce offspring with similar traits.