The **seat belt** is a crucial element of **passive safety** in vehicles, designed to protect all occupants in the event of an accident. Its use is mandatory for both the **driver** and **all passengers** of a car, on any type of road, whether urban or interurban. This is because the seat belt helps keep people in their seats and prevents them from being ejected from the vehicle during an impact or rollover, significantly increasing the chances of surviving an accident.

There are some specific exceptions where wearing a seat belt is not mandatory, such as when drivers are parking in reverse or taxi drivers on duty. However, these exceptions do not apply on roads, highways, or motorways, where the use of the seat belt is always mandatory. Additionally, people with medical or disability reasons may be exempt, but they must have a medical certificate to justify it.

Therefore, the correct answer is that the use of the seat belt is mandatory for the **driver and all passengers of a car**.