Which feature separates humans from other primates? smaller brain size bipedalism use of tools complex social skills

Complex social skills.

Explanation:

Humans actually have larger brains than other primates, so smaller brain size is incorrect.

Other primates, such as chimpanzees, orangutans, and gorillas, all have the ability to use tools, so that is incorrect.

Some primates, such as chimps and baboons, do stand on their hind legs at times, so bipedalism does not separate humans from other primates.

So, complex social skills separate humans from primates.

Hope this helps!