Any time prior to receiving the first annuity payment, the member may elect to receive in lieu of such annuity, but payable in the same manner, an actuarially equivalent annuity in one of the following forms:

(1) A joint and survivorship annuity which shall continue after the death of the member to the death of the (a) member's spouse or (b) other designated beneficiary whose adjusted age in the calendar year in which the payment of the annuity commences is no more than ten years less than the attained age of the member in such calendar year;

(2) A joint and survivorship annuity which shall continue after the death of the member so that seventy-five percent of the amount of the member's monthly benefit under this option shall be paid monthly to the (a) member's spouse until his or her death or (b) other designated beneficiary whose adjusted age in the calendar year in which the payment of the annuity commences is no more than nineteen years less than the attained age of the member in such calendar year until his or her death;

(3) An annuity payable monthly during the remainder of the member's life with the provision that in the event of his or her death before one hundred twenty monthly payments have been made the monthly payments will be continued as provided in this section until a total of one hundred twenty monthly payments have been made;

(4) A joint and survivorship annuity which will continue after the death of the member to the death of the (a) member's spouse or (b) other designated beneficiary whose adjusted age in the calendar year in which the payment of the annuity commences is no more than ten years less than the attained age of the member in such calendar year but which annuity shall, upon the spouse's or designated beneficiary's death before the death of the member, be increased after such death for the remaining life of the member so that the monthly benefit equals the monthly benefit which would have been payable to the member had the member selected the normal form of the formula retirement annuity specified in section 79-9,100; or

(5) A joint and survivorship annuity which shall continue after the death of the member so that fifty percent of the amount of the member's monthly benefit under this option shall be paid monthly to a designated beneficiary until his or her death.

For purposes of the annuities provided in subdivisions (1), (2), and (4) of this section, a designated beneficiary's adjusted age means the attained age of the designated beneficiary in the calendar year in which payment of the annuity commences plus the number of years, if any, by which the member's attained age in the calendar year in which payment of the annuity commences is younger than seventy years.

Each of these actuarially equivalent annuities, except for the form provided in subdivision (3) of this section, shall continue for a minimum of sixty months.

The amount of each monthly payment shall be the amount specified in the form elected by the member.

Whether the member elects the normal form or one of the optional forms of the formula retirement annuity, if the member and his or her designated beneficiary die before the specified monthly payments have been made, the remaining number of the specified payments shall be paid to the individual or individuals designated in writing, on forms prescribed by the system, by the last surviving of the member or the member's designated beneficiary and, if no such designation is made, to the estate of the last surviving of the member or the member's designated beneficiary. At the election of a beneficiary, a single sum payment which is the actuarial equivalent of the remaining monthly payments to be paid to such beneficiary may be paid in lieu of the annuity benefit otherwise to be provided under the normal form or the optional form described in subdivision (3) of this section.