Answer : 4. "Luni Basin"
Explanation :

1. Eastern Plains Region: Extension - The region situated east of the 50 cm rainfall line in Rajasthan is known as the Eastern Plains Region. This region is formed by rivers, which is why it is called a river basin or region.

2. The eastern plain is mainly divided into three parts.

- Chambal River Basin

- Banas River Basin

- Mahi River Basin