The secretary shall establish by rule categories of persons exempt, but the exemption applies only to the work requirements of the program: Provided, That a person who is exempt from the work requirements may nevertheless participate voluntarily in work activities. The categories of exemptions are limited to the following:
(1) Undocumented aliens and aliens under the five-year ban;
(2) Parents, or at state option on a case-by-case basis, anyone receiving supplemental security income;
(3) A parent who is providing medically necessary care for a disabled family member who resides in the home and is not a full-time student;
(4) Minor parents who are not head of household (spouses of the head of household); and
(5) Grandparents and other nonparental caretakers.