When entering a highway from a private road, what should you do before proceeding?

Answer: B — Stop and look both ways

Before entering a highway from a private road, it is important to come to a complete stop and look both ways before proceeding. This allows you to ensure that there are no fast-moving vehicles approaching and that it is safe to merge onto the highway. Looking right and left, as option A suggests, may not be sufficient as it does not account for vehicles coming from both directions. Checking the speed limit, as option C suggests, is important but not necessary before merging onto a highway. Flashing your lights, as option D suggests, is not a recommended action as it can distract other drivers and potentially cause accidents. Thus, the correct answer is to come to a complete stop and look both ways before proceeding onto a highway from a private road.