In general, when a ramp metering light is green: a. one car may advance b. traffic on the expressway is light c. traffic may proceed until the light turns red

Option 'C' is correct.

Explanation:

In general, when a ramp metering light is green, it means traffic may proceed until the light turns red.

Ramp meters are the device that control the traffic conditions.

As we know the basic rule of ramp meters:

1) Red light means the traffic must be stopped for a while.

2) Yellow light means the traffic needed to get ready to proceed when there is green light.

3) Green light means the traffic may proceed until it becomes red again.

Hence, Option 'C' is correct.