What endorsement must be attached to policies for all motor carriers under the Motor Carrier Act of 1980?

The endorsement that must be attached to policies for all motor carriers under the Motor Carrier Act of 1980 is the MCS-90. This endorsement is critical because it provides the necessary proof of financial responsibility for motor carriers engaged in interstate commerce. The MCS-90 ensures that these carriers have liability coverage for damages that occur during the transportation of goods, particularly in relation to environmental damage and bodily injury.

The inclusion of the MCS-90 endorsement helps ensure that anyone who is harmed by the operations of a motor carrier can receive compensation, regardless of whether the carrier's insurance policy would ordinarily cover that specific claim. This aligns with the federal regulations intended to promote safety and responsibility in the trucking industry, helping to standardize requirements across states.

The other options listed do not pertain to the requirements established under the Motor Carrier Act of 1980 and do not serve the same role in guaranteeing financial responsibility for motor carriers. Thus, the MCS-90 endorsement stands out as the legally mandated endorsement for complying with federal motor carrier regulations.