To solve this problem, we need to find the distance travelled by train.
Given that Dinesh travelled 1200 km by air, which formed 2/5 of his trip, we can find the total distance of his trip by multiplying 1200 km by 5/2.
Total distance of the trip = (1200 km) * (5/2) = 3000 km.
Next, we are told that one-third of the whole trip, Dinesh travelled by car. To find the distance travelled by car, we can multiply the total distance of the trip by 1/3.
Distance travelled by car = (3000 km) * (1/3) = 1000 km.
Finally, the distance travelled by train is the remaining distance after subtracting the distance travelled by air and the distance travelled by car from the total distance of the trip.
Distance travelled by train = Total distance of the trip - Distance travelled by air - Distance travelled by car
= 3000 km - 1200 km - 1000 km
= 800 km.
Therefore, the distance travelled by train is 800 km, which corresponds to option C.