The question is asking about which among the provided options directly joins the Ganga river.

Option 1 is Chambal. This river joins the Yamuna, not the Ganga directly. So it`s incorrect.

Option 2 is Son. It is a major and one of the largest tributaries of the Ganga, joining it at the city of Patna, in the state of Bihar. Hence, this is the correct option.

Option 3 is Betwa. This river merges with the Yamuna river at the city of Hamirpur, in Uttar Pradesh. It doesn`t join the Ganga directly, making it an incorrect option.

Option 4 is Ken. This river merges with the Yamuna near Banda, Uttar Pradesh. Like the Chambal and Betwa, it does not join the Ganga directly, so it is also incorrect.