The correct interpretation of the function \( y = 99x + 15 \) is:
- \( y \) represents the total cost.
- \( x \) represents the number of months of service.
- $99 is the installation fee (a one-time fee).
- $15 is the service charge per month.
So, the correct answer is:
D. Y is the total cost, x is the number of months of service, $99 is the installation fee and $15 is the service charge per month.