What is the function of a part in duct systems that directs and spreads air?

The correct choice is the diffuser because it plays a crucial role in duct systems by directing and spreading air efficiently within a space. A diffuser is designed to distribute air evenly in a room, ensuring that the airflow is gentle and covers a larger area, which enhances comfort and maintains temperature consistency. This component helps to reduce drafts and noise associated with the air movement.

In contrast, the other options serve different specific functions within HVAC systems. A blower is responsible for moving air through the ductwork but does not itself distribute or spread the air in the living space. A vent typically refers to an opening in a duct system where air enters or exits, but it does not actively spread or direct air like a diffuser. The return is part of the system that brings air back to the HVAC unit for recirculation and is not involved in the distribution of air within a room.