The correct answer is option 4, Son.

The question is asking about the river that joins the Ganga directly, while the other three rivers join the Yamuna.

Option 1: Betwa - This river does not directly join the Ganga. Instead, it joins the Yamuna river.

Option 2: Ken - This river also does not directly join the Ganga. It also joins the Yamuna river.

Option 3: Sind - Similar to the previous options, this river does not directly join the Ganga. It joins the Yamuna river as well.

Option 4: Son - This is the correct answer. The Son river directly joins the Ganga, making it the only river among the four options to do so.

Therefore, option 4, Son, is the correct answer.