Which of the following is NOT a method of relaying information visually?

The correct answer is C, the whistle, because it is primarily an auditory signal rather than a visual method of communication. It is designed to produce sound to alert individuals or convey instructions, making it ineffective for visual information relay.

In contrast, the other options represent methods that can convey information visually. For instance, direct telephone communication can include visual aids, such as charts or diagrams, when used in conjunction with telecommunication tools. Mobile Data Terminals (MDTs) provide visual information through screens displaying data, maps, and other relevant graphics. Television overrides can broadcast visual information to larger audiences through the medium of TV screens, which can be crucial during emergencies or public announcements. Thus, the whistle stands out as it lacks the capability to convey information visually.