The correct answer is (d) 15 km. West.
Rajesh goes 20 km towards South. After this he goes 10 km after turning to his left. In the end after turning to his left he goes 20 km. This means that he is now 20 km to the west of his starting point.
Option (a) is incorrect because Rajesh is not 15 km to the east of his starting point. He is 15 km to the west of his starting point.
Option (b) is incorrect because Rajesh is not 10 km to the east of his starting point. He is 15 km to the west of his starting point.
Option (c) is incorrect because Rajesh is not 20 km to the east of his starting point. He is 15 km to the west of his starting point.