What must a tour guide show when asked for their identification?

A tour guide must show their tour guide license when asked for identification. This license serves as a formal verification of the tour guide's credentials and demonstrates that they have met the required training and regulatory standards necessary to legally conduct tours in the area. It is an important credential that establishes trust and ensures that the tour guide is knowledgeable about the history, culture, and attractions of the location they are guiding.

The other options do not serve the same purpose. A payment receipt is a financial document that does not confirm the tour guide's qualifications. A promotional flyer may provide information about the tours offered, but it does not validate a person’s authority to conduct those tours. Customer testimonials, while valuable in reflecting past experiences, also do not serve as official identification or proof of certification. Therefore, the tour guide license is the appropriate and necessary form of identification in this context.