The correct answer is (b) an early human species.

Denisovans were an extinct group of humans that lived approximately 30,000 to 50,000 years ago. They are known only from a few fragmentary fossils discovered in a Siberian cave called Denisova Cave. The DNA of modern humans also shows evidence of interbreeding with Denisovans. This interbreeding is thought to have occurred when modern humans migrated out of Africa and into Asia, where they encountered Denisovans.

Studies of the Denisovan genome suggest that they were a distinct group of humans, separate from both Neanderthals and modern humans. They likely had a range of physical and behavioral adaptations that allowed them to survive in their environment, although much is still unknown about their biology and behavior.