With increase in speed of the traffic stream, the minimum spacing of vehicles A. increases B. decreases C. first decreases and then increases after reaching a minimum value at optimum speed D. first increases and then decreases after reaching a maximum value at optimum speed

The correct answer is: B. decreases.

The minimum spacing of vehicles decreases with increase in speed of the traffic stream. This is because the faster the vehicles are moving, the less time they have to react to changes in the traffic conditions. As a result, they need to leave more space between themselves to avoid collisions.

Option A is incorrect because the minimum spacing of vehicles decreases with increase in speed.

Option C is incorrect because the minimum spacing of vehicles does not first decrease and then increase after reaching a minimum value at optimum speed.

Option D is incorrect because the minimum spacing of vehicles does not first increase and then decrease after reaching a maximum value at optimum speed.