Assertion: Darwin"s finches show a variety of beaks suited for eating large seeds, flying insects and cactus seeds.
Reason: Ancestral seed-eating stock of Darwin"s finches radiated out from South America main land to different geographical areas of the Galapagos Islands, where they found competitor-free new habitats.

Both Assertion (A) and reason (R) are correct and R is the correct explanation of A.

At Galapagos Islands, Darwin observed that the finches had different types of beaks suited for eating large seeds, because it was the ancestral seed-eating stock of Darwin's finches radiated out from South American mainland to different geographical areas of the Galapagos Islands, where they found different types of food and competitor free new habitats.