Final answer:
It will take 6 months for the balance to be less than zero.
Step-by-step explanation:
To determine how many months it will take for the balance to be less than zero, we need to subtract the service fee of R99 from the balance each month until the balance reaches zero or becomes negative.
Starting with a balance of R500, after the first month, the balance will be R500 - R99 = R401.
By continuing this subtraction each month, we can create a table to track the balance:
MonthBalance1R4012R3023R2034R1045R56-R94
As we can see from the table, the balance becomes less than zero in the 6th month. Therefore, the correct option is a) 6 months.