In the function \( y = 99x + 15 \), \( y \) represents the total cost, \( x \) represents the number of months of service, \( 99 \) represents the service charge per month, and \( 15 \) represents the one-time installation fee.
Therefore, the correct interpretation is:
C. \( y \) 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.