From which group of animals are the mammals generally believed to have most recently evolved?

Explanation

The mammals are believed to have evolved from Therapsid (mammal-like) reptiles some 220 million years ago in the Triassic period, before the first birds.

So, the correct answer is 'Reptiles'

A  in the evolution of life occurred when mammals evolved from one lineage of reptiles. This transition began during the Permian (286 - 248 million years ago) when the reptile group that included  gave rise to the "beast-faced" therapsids. (The other major branching, the "lizard-faced" sauropsids, gave rise to birds and modern reptiles). These mammal-like reptiles in turn gave rise to the cynodonts e.g.  during the Triassic period.