Final answer:
In the function C(x) = 0.25x + 34, which represents car rental cost based on miles driven, the independent variable is x, representing the number of miles, with its units being miles.
Step-by-step explanation:
The question involves the function C(x) = 0.25x + 34, which represents the cost in dollars of renting a car for a day based on the number of miles driven. In this context, the independent variable is the variable x, which stands for the number of miles driven. This is because the independent variable is the one you can change or control. In the case of this function, the miles driven (x) determine the cost (C(x)), thus making it the independent variable. The units for the independent variable x are miles, since that's how the distance driven is measured. Therefore, the correct choice is a. Independent variable: x; Units: miles.