The correct answer is option (B) eocene era.
The eocene era is one of the divisions of the geological time scale characterized by the little or no ice with a small difference in temperature from the equator to the poles. It began 56 million years ago and the amount of oxygen was high compared to the previous era. Green house gases like methane and carbon dioxide were also present in the atmosphere. The oldest fossil evidences of the mammals appear during eocene period. It was characterized by the presence of the early primates, whales, horses and early elephants.
Elephants are the members of the family Proboscidae and the first proboscidaen is said to evolve during the late eocene era, about 35 million years ago. It was called the Paleomastodon with the size of a cow and had tusks and a trunk.
