c. Chordates.
Mammals are a group of animals that belong to the phylum Chordata, which also includes fish, birds, reptiles, and amphibians. Within the chordates, mammals belong to the class Mammalia, which is further divided into various orders, families, genera, and species.
a. Animals is too broad of a group and encompasses many different phyla, including Chordata (which contains mammals), as well as many other phyla that do not contain mammals.
b. Homnids refers to a family of primates that includes modern humans and our extinct relatives, but this taxon is too specific and is not inclusive of all mammals.
d. Primates refers to an order of mammals that includes humans, apes, monkeys, and lemurs, among others. While mammals are part of this group, it is not inclusive of all mammals, since there are many other groups of mammals that are not primates.