Question

what does it mean on a cladogram that modern birds and reptiles share a common ancestor. select the piece of evidence that best supports this idea.
reptiles are also related to mammals.
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.

Explanation:

A derived characteristic (a trait that evolved in a group and is passed down) that first evolved in reptiles and is present in birds (feathers) shows a connection. If reptiles had the precursor of feathers (a derived trait) and birds have feathers, it implies a common ancestry. The other options either talk about mammals (not relevant to bird - reptile common ancestor) or just about bird evolution (not about the link to reptiles).

Answer:

Feathers are a derived characteristic that first evolved in reptiles.