Which of the following is a possible reason for migration?
a. microevolution
b. diminished food supply
c. increased ecosystem stability
d. macroevolution

The correct answer is option B, that is, diminished food supply.  

Some of the animals move to comparatively short distances in search of food or more favorable breeding or living conditions. However, the majority of animals, which migrate do so in order to find more habitable conditions and food. The phenomenon can be witnessed in almost all the prime animal groups like mammals, birds, amphibians, fish, reptiles, crustaceans, and insects.  

i believe the answer would be b. hope it : )