The legal maximum length of a single vehicle is:
Answer: A — 23 metres
The correct answer is A. The legal maximum length of a single vehicle is 23 metres. This means that a vehicle cannot exceed this length to be considered legal. The other options are incorrect because they either fall short of the maximum length (D is 18 metres and B is 20 metres) or exceed it (C is 25 metres). Therefore, A is the correct option.