Final answer:
The order of mammals with the second greatest number of species worldwide is Chiroptera, the bats. They are unique among mammals for their true and sustained flight capability, and with about 1,000 species, they make up a significant portion of placental mammal diversity.
Step-by-step explanation:
The order of mammals with the second greatest number of species worldwide is Chiroptera, the bats. Among the Eutherians, or placental mammals, the two largest orders are Rodentia (rodents) - with about 2,000 species - and Chiroptera (bats) - with about 1,000 species. Thus, the correct answer is B) Bats. These orders collectively constitute approximately 60 percent of all placental mammal species.
Some notable characteristics of bats include their ability to achieve true and sustained flight, thanks to their forelimbs that form webbed wings. They are the only mammals with this capability.
Aside from bats, other significant orders of mammals include Rodentia (rats, mice, squirrels, and others), Carnivora (cats, dogs, bears, etc.), and Primates (humans, apes, monkeys). Each of these groups has adapted and evolved uniquely to succeed in various environments across the globe.