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A prohibitive sign is specifically designed to indicate that certain actions are not allowed. The description of a white square with a red circle and a diagonal red bar perfectly embodies the characteristics of a prohibitive sign. This design universally signals "no" or "not permitted," making it easily recognizable to drivers and pedestrians. The red circle indicates prohibition, while the diagonal bar reinforces the message that the action depicted is not allowed.

In contrast, the other options do not correctly represent prohibitive signs. A solid yellow background often signifies caution, while a green circular shape generally indicates permissible actions or guidance, such as directional information. Lastly, tall white rectangular signs typically provide information about regulations or guidelines, such as speed limits or instructions, rather than prohibitions. This is why the identification of a prohibitive sign is accurately represented by the characteristics described in the correct choice.