Which factor is most likely to contribute to the evolution of a species?

a stable environment in which members of the species live
genetic variation within the species
the inability of species members to adapt
reproduction difficulties for species members

genetic variation within the species

Explanation:

Having genetic variation allows for natural selection to act on the differential survival abilities and reproductive fitness of organisms within the species. A stable environment, inability of adaptation and reproduction difficulties all result in the inability for natural selection to act selectively within the species to allow for evolution. In these three case, selection pressures may result in the extinction of the species rather than natural selection and evolution.