Which of the following is excluded under additional agreements of the insurer in section C?

The option regarding customs duties on a new vehicle purchased in the USA being excluded under additional agreements of the insurer is correct because many auto insurance policies typically do not cover costs associated with importing vehicles, such as customs duties. These fees are considered a transactional cost instead of a loss or liability that arises from the operation of the vehicle itself.

In contrast, aspects like paying general average for other persons' autos, salvage costs for an insured vehicle after damage, and fire department charges for automobile fires relate directly to insurable events or claims arising from the usage of the vehicle. These are usually encompassed within the additional agreements of an insurer, as they are associated with mitigating loss or liability stemming from automotive incidents. Consequently, they are typically covered in various auto insurance policies to provide comprehensive protection for policyholders.