The correct answer is: a) Indus River
The Indus River is the longest river in Pakistan and the fifth-longest river in Asia. It is also the primary source of water for Pakistan and is vital for irrigation, drinking water, and hydroelectric power. The Indus River forms a natural boundary between Punjab (India) and Punjab (Pakistan) for about 100 miles.
The Sutlej River is the longest tributary of the Indus River. It originates in the Himalayas in Tibet and flows through India and Pakistan. The Sutlej River is not a natural boundary between Punjab (India) and Punjab (Pakistan).
The Ravi River is a tributary of the Indus River. It originates in the Himalayas in Himachal Pradesh, India, and flows through Punjab, India, and Pakistan. The Ravi River is not a natural boundary between Punjab (India) and Punjab (Pakistan).
The Beas River is a tributary of the Indus River. It originates in the Himalayas in Himachal Pradesh, India, and flows through Punjab, India, and Pakistan. The Beas River is not a natural boundary between Punjab (India) and Punjab (Pakistan).