The correct answer is option 4 - Ambala. Ambala is a city in the state of Haryana, India, and it is located in the region that acts as a water divide between the Ganga and Indus river systems. A water divide, also known as a watershed or water parting, is an elevated area that separates two drainage basins or systems. In this case, it separates the basins of the Ganga river and the Indus river. The Ganga river system is one of the major river systems in India, while the Indus river system is primarily located in Pakistan. Haridwar, mentioned in option 1, is a city located on the banks of the Ganga river but it is not a water divide. Namchobarva, mentioned in option 2, and Alakananda, mentioned in option 3, are not relevant to the water divide between Ganga and Indus river systems.