Final Answer:
The period in which mammals first appear is C) Cretaceous
Step-by-step explanation:
Mammals first appeared during the Cretaceous period, which lasted from approximately 145 to 66 million years ago. This period is notable for the diversification of mammals and other life forms.
The Pleistocene, Carboniferous, and Jurassic periods came later in geological time, each with its own distinct events. Cretaceous known for significant developments in the evolution of various life forms, including the diversification of mammals.
Therefore, option C) Cretaceous is correct answer.