What is the speed limit for trailers being towed on the motorway?

Answer: B — 60mph

The correct speed limit for trailers being towed on the motorway is 60mph. The other options are incorrect because A and D are too low and violate the general speed limit of 70mph on motorways, and C is not a common speed limit for motorways. Generally, trailers are not allowed to travel at the same speed as cars, so B is the most plausible answer.