The following persons may be adopted:
(1) A minor.
(2) An adult under any one of the following conditions:
a. He or she is totally and permanently disabled.
b. He or she is determined to be mentally retarded.
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.