The following persons may be adopted:
(1) A minor.
(2) An adult under any one of the following conditions:
a. He or she is an individual with a total and permanent disability.
b. He or she is determined to be a person with an intellectual disability.
c. He or she consents in writing to be adopted and is related in any degree of kinship, as defined by the intestacy laws of Alabama, or is a stepchild by marriage.
d. He or she consents in writing to be adopted by an adult man and woman who are husband and wife.