The correct answer is C. 45 km/h.
The maximum speed of a passenger train attached with a four-wheeler wagon is 45 km/h. This is because the four-wheeler wagon is not designed to travel at speeds above 45 km/h. If the train were to travel at a speed above 45 km/h, the four-wheeler wagon could become unstable and could derail.
Option A is incorrect because the maximum speed of a passenger train attached with a four-wheeler wagon is 45 km/h, not 70 km/h.
Option B is incorrect because the maximum speed of a passenger train attached with a four-wheeler wagon is 45 km/h, not 65 km/h.
Option D is incorrect because the maximum speed of a passenger train attached with a four-wheeler wagon is 45 km/h, not 75 km/h.