After reaching the milk booth, Rohan should take a left turn in the West direction.

Explanation:
To determine the direction in which Rohan should take a left turn after reaching the milk booth, we need to understand the concept of directions and left turns.

- Directions: Directions are the different ways in which we can move or face. The four main directions are North, South, East, and West. These directions are represented by the four cardinal points on a compass.
- Left Turn: A left turn is a movement or change in direction from the current path towards the left side.

Now, let's analyze the given options and determine the correct answer:

a) East: If Rohan takes a left turn towards the East, he will be moving in the opposite direction, away from the milk booth. This is not the correct answer.

b) West: If Rohan takes a left turn towards the West, he will be moving in the direction perpendicular to the milk booth. This is the correct answer as it ensures Rohan is still heading towards the milk booth.

c) North: If Rohan takes a left turn towards the North, he will be moving in a direction that is parallel to the milk booth. This means he will be moving away from the milk booth. Therefore, this is not the correct answer.

d) South: If Rohan takes a left turn towards the South, he will be moving in a direction that is opposite to the milk booth. This is not the correct answer.

Therefore, the correct answer is option 'b) West'. Rohan should take a left turn towards the West after reaching the milk booth to continue moving in the right direction.