The correct option is D between
In the given sentence the train connects the two cities; therefore the preposition used should denote the same. ‘Between’ is used when we are speaking about two places and the connection that is created between these two entities. In the given context, ‘between’ would mean that the train runs across these two cities. Therefore option D is correct. On the other hand, the other options are unsuitable since they change the meaning of the sentence. ‘From’ is used to speak about a starting point, hence when we use ‘from’ it would mean that the train starts from both Mumbai and New Delhi, separately. Whereas when we use ‘for’ it would mean that the reason for the train running is so that people can reach Mumbai and New Delhi. This would be inappropriate since one train will not help in reaching two entirely different cities. Similarly, ‘towards’ means to travel in the direction of something which once again is wrong since Mumbai and New Delhi are two different cities situated in geographically different areas. Thus options A, B, and C are incorrect.