The correct answer is - d. Jurassic.
The first true mammals appeared in the period known as the Jurassic, at its begging, around 199 million years ago, though there's disputes that maybe the mammals appeared few million years earlier, in the late Triassic period. The mammals appeared just 10 million years apart from the dinosaurs, and they lived along side them. Most of the early mammals were shrew-like animals, both in size, and behavior, being insect-eaters in general. But not all were so harmless, as there has been found an ancient mammal that was the size of smaller dog, which was a carnivore that was preying on the smaller dinosaurs.
After the period of the dinosaur domination abruptly came to an end around 66 million years ago, the mammals took the opportunity and filled in the empty niches in the ecosystem around the globe, thus quickly becoming the dominant animals on the planet.
