Detailed Solution:

The correct answer is “Cenozoic”.

Humans appeared during the Cenozoic Era, specifically in the Quaternary Period, which is the most recent period of this era.

The Cenozoic Era began about 66 million years ago and continues to the present day.

It is known as the "Age of Mammals" and saw the evolution of modern animals and the emergence of Homo sapiens (modern humans) roughly 300,000 years ago.