The **seat belt** is a crucial element for the protection of vehicle occupants in the event of an accident. Its main function is to keep people in their seats and prevent them from being ejected from the vehicle during an impact or rollover. For the seat belt to be effective, it must be properly fastened and adjusted to the body.

- **Shoulder strap**: It should pass correctly over the collarbone and the center of the chest. If it is incorrectly positioned, such as over the neck or a breast, it can cause serious injuries. Additionally, if placed over the shoulder, the belt may slip and lose effectiveness.

- **Lap belt**: It should be over the hip bones, not over the stomach, to avoid serious internal injuries in the event of an accident.

A poorly adjusted seat belt can cause the **submarining effect**, where the body slides under the belt during an accident, increasing the risk of injury. To prevent this, it is essential that the belt is well adjusted and that the seat is not too reclined backward.

In summary, if the seat belt straps are not properly stretched and adjusted, the belt **loses effectiveness** and can cause injuries during an accident, confirming that the correct answer is the first option.