18. The employee may, at any time before he retires, elect to receive, instead of the life annuity provided for in section 14, an annuity arising out of the amount of the pension credit payable, in one of the following methods:(a) a life annuity the payment of which is guaranteed for a period of 5, 10 or 15 years;
(b) a life annuity the payment of which continues in favour of the surviving spouse on the death of the employee;
(c) a life annuity the payment of one-half of which continues in favour of the surviving spouse on the death of the employee.