Final answer:
The defining feature of humans is a distinct trajectory of biocultural adaptation that involves language capabilities and complex cultural development.
Step-by-step explanation:
The defining feature that defines humans among primates is c) a distinct trajectory of biocultural adaptation. Primates, including humans, showcase unique physical features enhancing dexterity and a strong emphasis on vision rather than smell. However, humans evolved with extraordinary capacities like the invention and use of spoken language, enabling the development of complex cultures. Unlike other primates, humans have a notable history of technological advancement and cultural diversity, stemming from our biological and social adaptability.