What must be advised to customers regarding their travel documents?

In the travel industry, it's crucial to communicate to customers that entry requirements can change depending on various factors such as destination policies, the traveler's nationality, and current regulations, even if all documentation appears to be complete. This means that travelers should always verify the latest entry requirements for their destination before departure, as government regulations and acceptance criteria can shift frequently.

This proactive approach helps to prevent issues upon arrival, including denied entry, additional fees, or being required to return home. Ensuring that customers are aware of this variability encourages them to take responsibility for their travel arrangements and documents, fostering a smoother travel experience. The other options do not reflect the reality of travel documentation, as they either inaccurately assure travelers of guaranteed acceptance or undervalue the importance of documents in the travel process.