Final answer:
A sign with white letters on a red background typically signifies stop or prohibition, such as a Stop or Do Not Enter traffic sign, and is used to capture attention and convey urgent commands for safety.
Step-by-step explanation:
When one asks, Which sign has white letters on a red background, a common answer would be traffic signage indicating stop or prohibition, such as a Stop sign or Do Not Enter sign.
In the context of traffic signals, this color scheme is designed to capture attention and convey an immediate command, urging the driver to take action for their safety and the safety of others. This design principle is not just applied to traffic signals but can also be seen in various forms of signage where there is a need to alert or instruct individuals quickly and clearly.