What defines a narrow body aircraft?

A narrow body aircraft is specifically characterized by having a single aisle in the passenger cabin. This design typically accommodates a smaller number of passengers compared to wide-body aircraft, which have two aisles and can handle larger capacities and wider seating arrangements. Narrow-body aircraft are commonly used for short to medium-haul flights, making them efficient for regional travel. They are favored for their ability to operate in smaller airports and for routes that do not require the capacity of larger planes, thereby focusing on economies of scale in air travel.

The other options pertain to characteristics of different types of aircraft. Wide-body aircraft with two aisles (option about two aisles) are generally designed for long-haul flights (another option), as they offer more space and comfort for passengers over longer durations. Additionally, while some narrow-body aircraft can carry cargo, the defining feature regarding passenger configuration is the single aisle.