C. Only when it's wet
When driving at night on a road with lanes, you must be able to stop in the full length of clear road you can see in front of you.
The reason for the difference between a laned and unlaned road is this:
If you are driving in a road with lanes, each vehicle should have its own lane. If you need to stop, it's unlikely that there will be another vehicle coming towards you in your lane. If you are driving on an unlaned road it's likely to be a narrower rural road. This means that vehicles coming towards you could be partially in your lane. You must be able to stop and the other vehicle must be able to stop, too, therefore the distance is halved.
Unlaned roads are often of poorer quality road surface, too, and therefore stopping distances are likely to be longer.
Remember that your stopping distance consists of your thinking distance plus your braking distance and that a vehicle you meet could possibly have a longer stopping distance or be being driven by a person who has a slower reaction time resulting in a longer thinking distance.
For more information about stopping distances, including all the factors that affect it, see this article.