What should be done when using a freeway on-ramp?

Answer: C — Speed up to the same speed of traffic

When using a freeway on-ramp, it is important to increase your speed in order to merge safely and efficiently with the flow of traffic. Stopping prior to merging, as suggested in option A, can be dangerous and disrupt the flow of traffic. Accelerating quickly, as suggested in option B, can also be dangerous and lead to loss of control. Turning on your hazard lights, as suggested in option D, is unnecessary and can confuse other drivers on the road. Therefore, option C is the most appropriate and safest action to take when using a freeway on-ramp.