The correct answer is (b) 21 years.
According to the Hindu Adoption and Maintenance Act, 1956, the adopter must be at least 21 years older than the child being adopted. This is to ensure that the adopter is mature enough to take care of the child and provide them with a loving and supportive home.
The other options are incorrect because they are not the minimum age difference required by law. Option (a) is incorrect because the minimum age difference is 21 years, not 18 years. Option (c) is incorrect because the minimum age difference is 21 years, not 16 years. Option (d) is incorrect because the minimum age difference is 21 years, not 14 years.
It is important to note that the minimum age difference may vary depending on the adoption law in the country where the adoption is taking place. It is always best to check with the relevant authorities to find out the specific requirements in your country.