Answer: Option C.
Explanation:
In this type of statistical problems you usually want to find the value of a certain parameter. In this case, the parameter is the average of the voltage of the whole year.
Many times it is difficult to record the values for the entire population (record the voltage every day), and that is why you usually take a sample of the population (some of the days) and find the average voltage based on it to estimate the parameter.
This value that estimates the parameter is called statistic, therefore the correct option is C.