A person who is solely a National Guard member or who retires from membership in this system solely as a National Guard member may elect to receive service credit under either of the following:
(a) The National Guard member elects to receive service credit pursuant to Section 20750, 20751, 20751.5, or 20753.
(b) The National Guard member elects to receive service credit pursuant to a provision that requires the member to pay contributions as described in Sections 21050 and 21052.