Which color of stoplight must be used in combination with the taillight?

The correct choice indicates that a red color of stoplight must be used in combination with the taillight. This is grounded in traffic safety standards and regulations. The red stoplight signals to other drivers that the vehicle is stopping or has come to a halt, making it essential for preventing accidents and ensuring safety on the road.

The taillight serves the purpose of providing visibility to vehicles behind, and when combined with a red stoplight, it effectively communicates to drivers that the vehicle is slowing down or stopped. This setup is crucial, especially in low-light conditions or during nighttime, as it alerts other drivers to the presence of a vehicle that is not in motion.

In contrast, amber, green, and blue lights serve different purposes. Amber lights typically signal caution or a slow-down requirement, green lights indicate that it is safe to proceed, and blue lights are usually reserved for emergency vehicles. Using these colors in conjunction with the taillight would not convey the necessary warning of a vehicle's stop, which is why red is the only appropriate color in this context.