Answer:
C. The number of GB data used.
Explanation:
Independent variable is the variable that is changed or manipulated in an experimental scenario. The independent variable is changed in order to effect response in another variable called Dependent variable, which is dependent on the independent variable.
According to this question, a person gets a smartphone at a certain cost and signs up for a monthly wireless plan with a fixed base price plus a charge per GB of data used. One would observe that the variable changed here is the "Number of GB of data used", hence, it is the INDEPENDENT VARIABLE. The charge per GB of data used is dependent on the number of GB of data used hence it is the dependent variable.