The Siachen Glacier`s melting waters are the main source of the Nubra River. It is important to understand that the Siachen Glacier is located in the Eastern Karakoram Range in the Himalayas, spanning across the line of control between India and Pakistan. The glacier is known for its significant size and is considered the longest glacier in the Karakoram Range. As it melts, the water flows down and contributes to the formation of several rivers, including the Nubra River. The Nubra River is a tributary of the Shyok River, which eventually joins the Indus River. It is important to note that the other options mentioned in the question (Beas, Sutlej, Shylok) are incorrect and can be eliminated.