Which of the following is called Sewall Wright effect?

Answer: Option A

Genetic drift is called the Sewall Wright effect.
This effect says that when small populations of a species are isolated, out of pure chance the few individuals who carry certain relatively rare genes may fail to transmit them.
Genetic drift is the change in frequency of an existing gene variant in the population due to random chance. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation.