What does a no-passing zone sign indicate?

A no-passing zone sign indicates that drivers must not pass other vehicles in that area. These signs are typically placed in locations where visibility is limited, such as on hills or curves, to enhance safety by preventing accidents that may result from vehicles attempting to pass when they can't see oncoming traffic. The intent of this sign is to reduce the risk of collisions by indicating that the area is not safe for overtaking other vehicles.

The other options suggest incorrect actions or conditions related to passing. For instance, the idea that drivers must pass vehicles immediately contradicts safe driving practices by encouraging risky behavior. Similarly, stating that passing is allowed only at night does not pertain to the function or authority of a no-passing zone sign, which applies at all times. Lastly, while the speed of vehicles may be affected by the presence of a no-passing sign, the sign itself does not direct all vehicles to slow down but rather advises drivers against passing. Therefore, option C accurately reflects the purpose of the no-passing zone sign.