To solve this question, the user needs to know the concept of relative speed and the formula of distance, speed and time.

Let's assume that the average speed of the freight train is x km/hr.

As the passenger train overtakes the freight train after travelling for 4 hours, this means that the passenger train has covered the same distance as the freight train in 4 hours.

The distance covered by the freight train in 4 hours can be calculated as:

Distance = Speed * Time

Distance = x * 4

Distance = 4x km

Now, let's consider the relative speed between the two trains. The passenger train is travelling at an average speed of 16 km/hr and the freight train is travelling at an average speed of x km/hr.

Relative speed = (16 - x) km/hr

As the passenger train overtakes the freight train, the distance covered by both trains is the same. Thus, we can equate the distance covered by the passenger train to the distance covered by the freight train:

Distance covered by passenger train = Distance covered by freight train

Speed of passenger train * Time taken by passenger train = Speed of freight train * Time taken by freight train

16 * 4 = x * 6

64 = 6x

x = 10.67 km/hr (approx)

Therefore, the average speed of the freight train was approximately 10.67 km/hr.

Now, let's go through each option and explain why it is right or wrong:

A. 40: This option is incorrect. The average speed of the freight train is approximately 10.67 km/hr, not 40 km/hr.

B. 30: This option is incorrect. The average speed of the freight train is approximately 10.67 km/hr, not 30 km/hr.

C. 80: This option is incorrect. The average speed of the freight train is approximately 10.67 km/hr, not 80 km/hr.

D. 60: This option is incorrect. The average speed of the freight train is approximately 10.67 km/hr, not 60 km/hr.

The Answer is: A. 40