Which of the following violations could lead to a downgrade in a carrier's safety rating?

Operating without a valid license is a significant violation that can directly lead to a downgrade in a carrier's safety rating. Licensing is fundamental to the legal operation of a commercial vehicle, as it ensures that the driver has met the necessary training and competency standards. A valid license indicates that the driver has the knowledge and skills required to operate the vehicle safely and comply with traffic laws. If a carrier operates with unlicensed drivers, it suggests a lack of commitment to safety regulations and can be viewed as a serious compliance issue. This can lead to increased scrutiny from regulators and result in a downgrade of the carrier’s safety rating due to the potential risk posed to the public and the liability it creates.

In contrast, while the other violations may also reflect safety concerns, they do not hold the same weight in terms of regulatory compliance and the direct implications for the carrier's legal operation. For example, driving with a passenger under 18, not using seat belts, and overloading the vehicle may indicate poor safety practices but do not explicitly violate licensing requirements that are critical for a carrier’s legal operational standards.