Explanation:

- The cost price (CP) of the book is Rs. 600.

- The shopkeeper earns a profit of 33 1/3% on the CP.

- Converting 33 1/3% into a fraction, we have 1/3, so the selling price (SP) is 4/3 of the CP.

- Calculate SP: 4/3 * 600 = Rs. 800.

- A 20% discount is given on the marked price (MP).

- Let the marked price be X.

- With a 20% discount, the selling price is 80% of the marked price: 0.8 * X = 800.

- Solve to find X: X = 800 / 0.8 = Rs. 1000.

- The discount is MP - SP = 1000 - 800 = Rs. 200.

Correct Answer: Option:1 - 200