The Dudhsagar Falls in Goa are formed by the river:

The Dudhsagar Falls are a four-tiered waterfall on the Mandovi river, in the Bhagwan Mahavir Wildlife Sanctuary and Mollem National Park, on the Goa-Karnataka border in the Western Ghats.

Features of Dudhsagar Falls:

• They are about 310 metres (1,017 feet) high, among the tallest waterfalls in India.
• The average width is about 30 metres.
• "Dudhsagar" means "sea of milk", because the falling water looks like a stream of milk, especially during the monsoon.
• The Londa-Vasco da Gama railway line passes across the face of the falls on a bridge, giving train passengers a spectacular view.
• The falls are at their best from June to September, though access is restricted during the heaviest rains for safety.

About the Mandovi river:

• It is also called the Mahadayi or Mhadei, especially in its upper course.
• It rises in the Bhimgad area of the Western Ghats in the Belagavi (Belgaum) district of Karnataka.
• It flows west through Karnataka and Goa and enters the Arabian Sea near Panaji, Goa's capital.
• It is about 77 to 81 km long.
• Together with the Zuari, it forms a network of waterways around the islands of Goa, which is important for shipping iron ore from the mines of Goa to Mormugao port.

Mahadayi water dispute:

• Goa, Karnataka and Maharashtra have had a long-running dispute over sharing the Mahadayi waters, especially over Karnataka's plan to divert water to the Malaprabha basin (the Kalasa-Banduri project).
• The Mahadayi Water Disputes Tribunal gave its award in 2018.

Rivers of Goa:

• Goa's major rivers include the Mandovi, Zuari, Terekhol, Chapora, Sal, Talpona and Galgibag.
• The Zuari is often described as the largest river of Goa by basin area.

Why not the other options?

• ✗The Zuari and Chapora are other rivers of Goa.
• ✗The Sharavati forms the Jog Falls in Karnataka.