Mammals are diphyodont which refers to the fact that
a. in adulthood, the post canine teeth can be replaced twice
b. they have a variety of tooth shapes that allow for them to process a wider variety of foods.
c. adults only have two types of teeth and so can process food differently in the front of the mouth and the back of the mouth
d. they have two successive sets of teeth including milk teeth and permanent adult teeth
e. they have a set number of incisors, premolars, and molars

The correct answer is D. They have two successive sets of teeth including milk teeth and permanent adult teeth

Explanation:

In biology, a diphyodont is an animal that has two different sets of teeth, this often means the animal has a set of baby teeth that are later replaced by permanent teeth, this occurs in most mammals including humans. Additionally, diphyodont differ from other types of animals that have either only one set of teeth (monophyodonts), for example, toothed whales or change their teeth permanently (polyphyodont), for example, crocodiles. Thus, mammals are diphyodont because "they have two successive sets of teeth including milk teeth and permanent adult teeth".