Which item must be included in each demo kit?

Including a seat belt in each demo kit is crucial because it directly pertains to passenger safety during flight. Demonstrating how a seat belt works is an essential part of pre-flight safety briefings, as it helps passengers understand how to secure themselves properly during takeoff, landing, and turbulence. Ensuring that passengers are familiar with fastening and unfastening their seat belts can significantly enhance their safety and comfort.

In the context of the other options, while emergency equipment such as exit signs, fire extinguishers, and first aid kits are important for overall safety, they are typically addressed in different contexts or demonstrations. The seat belt, however, is a universal requirement and is fundamental to passenger safety in airplane operations.