Tenn. Code § 17-532
The amount of any optional retirement benefit set forth in this section shall be according to option rates or actuarial bases adopted from time to time by the board.
Option A: 120 payments certain and life option: An option A benefit is a decreased retirement benefit payable for life with the first 120 payments guaranteed. Any guaranteed payments due after the death of the retired member shall be payable to his beneficiary.
Option B: joint and survivor option: An option B benefit is a decreased retirement benefit payable to the retired member for life which shall continue after his death to the surviving beneficiary for life in the same amount as that payable to the retired member.
Option C: modified joint and survivor option: An option C benefit is a decreased retirement benefit payable to the retired member for life which shall continue after his death to the surviving beneficiary for life in the amount of fifty per cent (50%) of the amount that was payable to the retired member.
Option D: social security option: An option D benefit is an increased retirement benefit payable to the retired member during his lifetime until his normal retirement date and a reduced retirement benefit payable thereafter for life in order to have a more level retirement income when such reduced retirement benefit is added to his primary insurance amount payable under the Social Security Act. The optional benefit shall be based on the retired members' estimated primary insurance amount payable under the Social Security Act as such act exists on his early retirement date.
The amount of any optional retirement benefit set forth in this section shall be determined on the basis of the actuarial assumptions that have been adopted by the board and are in use at the time of determination.
If the member is over age fifty-five (55) at date of determination, the optional factors used will be those which would have been applicable when the member was age fifty-five (55) but based on the actuarial assumptions in use at the time of determination.
T.C.A. § 17-532