What does a solid white line across a lane of traffic indicate?

Answer: D — Do not change lanes or cross the line.

A solid white line across a lane of traffic indicates that drivers are not allowed to change lanes or cross the line. This option is the correct answer because it accurately reflects the meaning of a solid white line. Option A is incorrect because it suggests merging with traffic in the adjacent lane, which is not allowed when there is a solid white line. Option B is incorrect because it implies waiting for a traffic light, which is not relevant to a solid white line. Option C is incorrect because it suggests changing lanes only if safe, but a solid white line indicates that changing lanes is not allowed at all.