Option (1) is incorrect: It is not a valid adoption.
Option (2) is incorrect: It is not voidable being void ab initio.
Option (3) is incorrect: The adoption is void being against the provisions of law.
Option (4) is correct: For a valid adoption, if the adoption is by a male and the person to be adopted is a female, the adoptive father should be at least 21 years older than the person to be adopted.