What is an invalid use of an official rental car at a TDY location?

The correct choice highlights that using an official rental car to visit relatives during non-duty days is considered an invalid use. Rental cars provided for official travel are intended solely for activities related to the official purpose of the trip. This includes attending meetings, training sessions, or traveling to the airport after completing official duties. When an individual uses the rental car for personal reasons, such as visiting family, it deviates from the intended use and guidelines set for official travel.

This clarity in purpose is crucial for maintaining compliance with travel regulations and ensuring that government resources are used for their intended official functions. Other mentioned uses fall within acceptable parameters as they directly relate to the official travel mission.