Answer : 4. "हिंदो"
Explanation :

1. The Indus River was called Hindu by the Iranians and Greeks about 2500 years ago.

2. The word Hind is derived from the Sanskrit word Sindhu. The word Sindhu means "ocean" or "big river".

3. The Indus River flows through parts of Pakistan and India. It is one of the longest rivers in the world