Which of the following is a primary source of genetic variation in populations?

Mutations are the primary source of genetic variation in populations because they introduce new genetic material into the gene pool. Mutations can occur in various ways, such as through changes in DNA sequences, insertions, deletions, or substitutions. This new genetic information can lead to new traits in individuals, contributing to the diversity within a population. As these mutations are passed on to offspring, they can affect the population's overall genetic structure, making mutations a crucial mechanism for evolution.

On the other hand, environmental resistance, selective breeding, and habitat fragmentation do not directly serve as sources of genetic variation. Environmental resistance refers to factors that limit population growth and may select for certain traits but do not create new genetic variations. Selective breeding is a human-directed process of choosing specific traits, which utilizes existing genetic variability rather than introducing new variation. Habitat fragmentation can influence genetic diversity by isolating populations but does not itself generate new genetic differences within those populations.