This sign means:

Answer: B — Traffic is entering from another road and you should drive cautiously while allowing additional space for the other drivers to merge safely into the traffic flow

This sign is usually seen when entering a busy highway or freeway and it means that there is an entrance ramp ahead where traffic from another road will be merging in. Drivers should drive cautiously and be prepared to slow down or stop to allow ample space for the merging traffic to merge safely into the flow of traffic. Option A is incorrect because it does not specify the reason for stopping. Option C is incorrect because keeping right is not the only thing that drivers should do when traffic is merging. Option D is incorrect because this sign does not prohibit overtaking, it is simply warning drivers to be cautious when merging traffic is present.