Which scenario indicates that vehicle 3 cannot proceed?

Answer: A — Traffic light is red

A red traffic light indicates that vehicles must stop and cannot proceed until the light changes to green. It serves as a clear signal for drivers to halt their progress at the intersection or traffic signal. This rule is fundamental to traffic safety and maintaining order on the roads, ensuring that vehicles do not enter intersections when it is unsafe. In contrast, if another vehicle has already moved or if there is a clear path, these conditions do not necessarily restrict vehicle 3's ability to proceed. Additionally, the presence of stationary vehicles only suggests that there may be no immediate traffic flow, but it does not remove the directive given by a red traffic light. Therefore, the key factor in this scenario that indicates vehicle 3 cannot proceed is the red traffic light.