(a) Except as provided by Subsection (b) or (c), a disability retirement annuity is an amount computed as provided by Section 839.102.
(b) The amount of a disability retirement annuity is not reducible because of the age of the retiring member.
(c) Instead of a disability retirement annuity computed as provided by Section 839.102, a retiring member may elect to receive an optional disability retirement annuity payable as provided by Section 839.103.
(d) A disability retirement annuity is payable for the duration of the retiree's disability. If a retiree who has selected an optional disability retirement annuity dies while receiving the annuity, the annuity is payable throughout the life of the retiree's designated beneficiary or for a guaranteed period after the date of retirement, depending on the option selected.