Explanation:

- The Payment of Wages Act, 1936 ensures timely payment of wages to employees and prevents unauthorized deductions.

- It applies to employees earning wages within a specific limit, which can be updated by authorities.

- Option 1: Rupees fifteen thousand - Previously, this was the limit under the Act but may have been revised.

- Option 2: Rupees eighteen thousand - Historically updated limit, but check current regulations.

- Option 3: Rupees twenty-one thousand - Most recent limit as per the latest amendments.

- Option 4: Rupees twenty-four thousand - Currently not the standard limit for the Payment of Wages Act, 1936.

Option 3: Rupees twenty-one thousand is correct.