Variations help in coping up with the changes in the environment. Hence variations promote the survival of the species.
New species are formed due to variation in the DNA for several generations. Asexual reproduction will not result in variation as there are no gametes involved. Movement of individuals from one habitat to another will not affect DNA change hence it cannot be the right answer.
Geographical conditions and environment
Geographical isolation of individual of a species leads to genetic drift. This limits the sexual reproduction of the separated population. This results in separated individuals reproducing among themselves. This leads to the formation of a new variation. Accumulation and transfer of these variations through generation will lead to the formation of new species.