The correct answer is option 2, Ghaghara. The river marked `x` in the given map is a tributary of the River Ganga.

Option 1, Gandak, is also a tributary of the River Ganga, but it is not the river marked `x` in the map.

Option 3, Kosi, is a river in Nepal and also a tributary of the River Ganga, but it is not the river marked `x` in the map.

Option 4, Son, is a river in the states of Jharkhand and Bihar in India. It is not a tributary of the River Ganga, so it is not the river marked `x` in the map.

Therefore, the correct answer is option 2, Ghaghara. Ghaghara is a river in northern India and a major tributary of the River Ganga. It has its origin in the Himalayas and merges with the River Ganga near the town of Chhapra in Bihar.