Correct Answer - Option 4 : Kolkata to Mumbai
Instead of calculating the amount for all the cities, let us go by options:
(1) Ahmedabad to Jaipur = Rs. 10
(2) Mumbai to Bangalore = Rs. 25
(3) Jaipur to Bangalore = Rs. 10
(4) Kolkata to Mumbai = Rs. 7
∴ The least expensive is from Kolkata to Mumbai.