Final answer:
Genetic variation within a population is mostly due to differences in the nucleotide sequences of DNA, arising from mutation and sexual reproduction, both contributing to the genetic diversity of a population.
Step-by-step explanation:
The majority of genetic variation among individuals within a population is due to differences in the nucleotide sequence of DNA. This variation arises because the majority of human genes have multiple alleles, or different forms, that can exist due to variations in individual DNA base pairs within these alleles. The genetic diversity observed in a population stems primarily from mutation and sexual reproduction, both of which can introduce new alleles into a population's gene pool.
Mutation is the ultimate source of new alleles, providing a raw material for evolution. However, the variation that has the potential for evolutionary change is that which is heritable, meaning it must be encoded within an individual's genes. During sexual reproduction, the combination of alleles inherited from both parents results in further variation among individuals.
Therefore, the correct answer to the students' multiple choice question is d. Differences in the nucleotide sequence of DNA, which is the underlying factor contributing to genetic variation within a population.