Under what condition can a taxi transport fortified wine or spirits?

The correct condition for a taxi to transport fortified wine or spirits is when there is a paying customer. This aligns with many alcohol beverage control laws that specify that the transportation of alcohol should occur in the context of a commercial transaction. A paying customer signifies that the transportation of the alcoholic beverage is part of a service being provided, which complies with regulations concerning the lawful transport of alcohol.

Transporting alcohol without a paying customer could raise concerns about the legality of the transaction and whether it adheres to local regulations governing the carriage of alcoholic beverages. In many regions, certain stipulations ensure that alcohol is only transported as part of a legitimate service, providing a safeguard against potential misuse, underage consumption, and other legal issues related to the transport of alcoholic products.

In contrast, the other options present conditions that either lack the necessary commercial context (like transporting without any limit or solely relying on the sobriety of the driver) or do not specifically require the presence of a paying customer, which is essential for compliance with many alcohol transport laws.