Which of the following provides the best example of selective breeding?

a
Scientists have noted that different varieties of birds on different islands have beaks that are suited to the birds available foods
b
Racehorses that run fastest are the ones that are allowed to produce offspring, leading to faster racehorses
c
Leopard seals captured in Antarctica have thicker layers of blubber than harbor seals captured in the Pacific Ocean
d
Hummingbirds have a long, narrow beak that enables them to reach food in the deepest part of the flower