Final answer:
Type B vent is a double-wall construction that is designed for venting non-condensing gas appliances, usually consisting of an aluminum inner wall and a galvanized steel outer wall.
Step-by-step explanation:
The question asks to identify what a Type B vent is. The correct answer is B. double wall. Type B vents are designed for venting non-condensing gas appliances. They have a double-wall construction that provides an air-insulated vent system. The inner wall is usually made of aluminum, while the outer wall is made of galvanized steel. This construction allows for a safer passage of exhaust due to the reduced clearances to combustibles and improved insulation compared to a single-wall vent.