Answer : 2. "50 cm Isohyets"
Explanation :

1. The eastern boundary of the western sandy plain is determined by the 50 cm isobar line. This line divides Rajasthan into two parts: the western sandy plains and the eastern plains.

2. The amount of rainfall in the western sandy plain is less than 50 cm. This area is part of the Thar Desert. The amount of rainfall in the eastern plains is more than 50 cm. This region is situated to the east of the Aravalli mountain range.