Question

Mammary glands are a shared characteristic among mammals and the oldest common ancestor of mammals. This is an example of a

Answer 1

B. synapomorphy

A synapomorphy is a shared apomorphy that distinguishes a clade from other organisms.
A clade also known as monophyletic group, is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single "branch" on the "tree of life".

Answer 2

The answer is B. Synapomorohy.