The correct answer is (d) 10 km.
The cyclist goes 30 km to the North and then turning to the East he goes 40 km. This means that he is now 40 km to the East of his starting point. Again, he turns to his right and goes 20 km. This means that he is now 20 km to the South of his starting point. After this he turns to his right and goes 40 km. This means that he is now 40 km to the West of his starting point. Therefore, he is 10 km from his starting point.
Option (a) is incorrect because the cyclist is not 25 km from his starting point. He is 10 km from his starting point.
Option (b) is incorrect because the cyclist is not 40 km from his starting point. He is 10 km from his starting point.
Option (c) is incorrect because the cyclist is not 20 km from his starting point. He is 10 km from his starting point.