What's the maximum length of a combination vehicle consisting of a motorcycle and a trailer?

In the photo a large touring motorcycle with two people on it is pulling a small luggage trailer. The maximum length of a combination made up of a motorcycle and a trailer is 4 m - answer B.

The Road Traffic Act states that the length of a vehicle combination made up of a motorcycle and a trailer must not exceed 4 m; the same limit applies to a moped, a bicycle and a cargo cycle with a trailer. The Regulation on the technical conditions of vehicles repeats this number among the permitted dimensions of vehicles. Four metres is not much - a large motorcycle alone is about 2.5 m long, so about 1.5 m is left for the trailer with its drawbar. The point is for the combination to stay manoeuvrable and stable: a longer trailer behind a motorcycle starts to "snake" at higher speed and makes overtaking difficult. Additional limits: the actual total mass of the trailer must not exceed the unladen mass of the motorcycle or 100 kg, and the combination may consist of at most two vehicles. The answers 5 m and 3 m do not match the provision.

Legal basis: Art. 62 para. 4a point 2 and para. 1 point 3 of the Road Traffic Act (Prawo o ruchu drogowym); § 2 para. 1 point 11 of the Regulation on the technical conditions of vehicles and the scope of their necessary equipment