Select the regulatory sign.

A regulatory sign is a type of traffic sign that indicates laws and regulations that drivers must follow. These signs inform drivers about the rules of the road, often dictating actions such as stopping, yielding, or the speed limit that should be adhered to.

The "STOP" sign clearly serves as a directive for drivers to come to a complete stop at an intersection. This sign informs drivers that they must halt their vehicle’s movement and assess the traffic situation before proceeding. It is a critical sign for maintaining safety at intersections where other vehicles or pedestrians may be present.

The other options, while they might convey important information, do not designate regulatory actions in the same way. For instance, the sign indicating "35 MPH" communicates a speed limit but is not as direct in its regulatory action as the "STOP" sign. Similarly, the "STOP SIGN AHEAD" is an advisory warning rather than a regulatory sign because it does not require a specific action. Thus, the correct answer identifies the direct regulatory nature of the "STOP" sign, which is essential for safe driving practices.