The correct answer is D. 1000 vehicles per hour.
The basic capacity of a traffic lane is the maximum number of vehicles that can safely pass a given point on a highway during a given time period. It is determined by a number of factors, including the speed of the vehicles, the number of lanes, and the presence of any obstructions.
In this case, the average center to center spacing of vehicles is 20 meters. This means that the average vehicle is 10 meters long. If the speed of the vehicles is 50 kmph, then each vehicle will travel 100 meters in 20 seconds. Therefore, the basic capacity of a traffic lane is 50 vehicles per minute, or 1000 vehicles per hour.
Option A is incorrect because it is the number of vehicles per day, not per hour. Option B is incorrect because it is the number of vehicles per hour, but it is based on an average center to center spacing of vehicles that is too small. Option C is incorrect because it is the number of vehicles per hour, but it is based on an average center to center spacing of vehicles that is too large.