The correct answer is - B. Beak size determines what the bird can eat.
The beak of a bird can come in many different shapes and sizes. It can be pointy, long, short, rounded, scythe-like, thin, thick... All of those shapes and sizes have a specific role, and that role is to enable the bird to feed itself with certain type of food source. Every food source requires certain type of beak in order for the bird to be efficient in getting its nutrition, so depending on hat the bird eats, we can easily see a pattern in the beaks, where birds that eat nuts have one strong and shorter beak, the ones that eat warms and insects have thin, pointy one, the predator birds have claw like, sharp beak...
