Question

it can be seen on a cladogram that modern birds and reptiles share a common ancestor. select the piece of evidence that best supports this idea
birds have evolved into many groups.
feathers are a derived characteristic that first evolved in reptiles.
birds have the derived characteristic of being warm blooded, like mammals.
reptiles are also related to mammals.

Explanation:

A derived characteristic that first evolved in reptiles and is shared (in a modified form) with birds (feathers) shows a common evolutionary origin. Birds evolving into many groups doesn't directly relate to a common ancestor with reptiles. Birds being warm - blooded like mammals is a characteristic shared with mammals, not reptiles. Reptiles being related to mammals doesn't directly support the bird - reptile common ancestor.

Answer:

Feathers are a derived characteristic that first evolved in reptiles.