Correct Answer - Option 2 : 10 years
According to the question:
Rs. 1000 invested in compound interest becomes Rs. 2000 in 10 years
The amount will double again in another 10 years.
So, the amount will become Rs. 4000 in another 10 years.
To earn another Rs. 2000 interest, it will take another 10 years
∴ The answer will be 10 years.