In: Biology
The correct Choice is:
A. gene flow, because otherwise independent differences cannot accumulate.
This will give rise to new species. When gene flow will not occur between two populations the newly arised characters will accumulate in the seperate populations and both populations will evolve into seperate populations.
Other choices are incorrect because:
B. If Natural Selection stops then all the members have the same opportunity to survive and as such there will be no chances to the individuals having some advantage to give rise to a new species.
C. Mutations give rise to new characters. Mutations which has positive effect on the survival or reproduction of the individual is selected against others. When these mutations accumulate in progeny. This will give rise to new species.
D. Sexual selection also leads to accumulation of the newly inherited characters which will lead to speciation.