The correct answer is option 2: Satluj. The River Beas, which flows through the states of Himachal Pradesh and Punjab, joins the Satluj River.

The River Beas originates in the Himalayas in Himachal Pradesh and travels through the Kullu Valley before entering the state of Punjab. It then flows through the state of Punjab and finally joins the Satluj River.

The Satluj River is one of the major tributaries of the Indus River. It originates in the Tibetan plateau and flows through the northern Indian states of Himachal Pradesh and Punjab before entering Pakistan and eventually joining the Indus River.

It is important to note that while the River Beas joins the Satluj River, it is not a tributary of the Indus River. The Indus River is formed by the confluence of the Satluj and Chenab Rivers, among others.

In summary, the River Beas joins the Satluj River, which is one of the major tributaries of the Indus River.