Which of the following is a valid exception to the seat belt law for adults?

The valid exception to the seat belt law for adults is the circumstance where the driver is reversing. In many jurisdictions, seat belt laws may have specific provisions regarding the use of seat belts while a vehicle is in reverse. This is often due to the short duration and the controlled environment in which the vehicle is moving backward, making the risk associated with not wearing a seat belt less than during regular driving situations.

In contrast, the other scenarios do not typically qualify as valid exceptions under seat belt laws. Holding a mobile phone, riding in a taxi, or having a pet in the vehicle generally requires all occupants to wear seat belts for safety purposes, as these situations do not provide a reasonable justification for not using this critical safety feature.