The correct answer is option 4, Sone.

The Ganga river system is composed of several tributaries. A tributary is a smaller river that flows into a larger river. In this question, we are asked to identify which of the following tributaries of the Ganga system flows northwards.

Option 1, Kosi, does not flow northwards. It is a river that flows from Nepal through the states of Bihar and Uttar Pradesh, eventually joining the Ganga river from the east.

Option 2, Ghaghara, does not flow northwards. It is a transboundary river that flows through Nepal and India, and it joins the Ganga river in India from the west.

Option 3, Gandak, does not flow northwards. It is also a transboundary river that flows through Nepal and India, joining the Ganga river from the west.

Option 4, Sone, is the correct answer. It is a river that flows through the states of Madhya Pradesh, Chhattisgarh, Bihar, and Uttar Pradesh before joining the Ganga river from the west. The Sone river flows in a north-northwest direction.