Which sign advises slower traffic to keep right?

Answer: A — white rectangle that says Slower Traffic Keep Right

The sign that indicates slower traffic should keep to the right is specifically designed to provide clear instructions to drivers about where to position themselves on the roadway. A white rectangle that states "Slower Traffic Keep Right" conveys this message unambiguously, serving to enhance safety and the flow of traffic. This type of sign is usually found on multi-lane roads, where faster-moving vehicles are encouraged to use the left lanes for passing, while slower vehicles should remain in the right lanes, effectively minimizing congestion and potential collisions. The use of a white rectangular shape is standard for regulatory signs, which is why this option correctly represents the sign's purpose. In contrast, other signs mentioned serve different functions: a yellow warning sign alerts drivers to upcoming hazards or conditions but does not regulate traffic flow, a red circular sign typically indicates prohibitions (like stop or yield), and a square speed limit sign informs drivers of the maximum (or minimum) speed allowed but does not pertain to lane usage for slower traffic.