Answer:
It's a statistic.
Explanation:
The diference between statistic and parameter is very simple. A parameter refers to a value that states something about a complete population that is under study. In practice, knowing parameter is really hard, because in most cases the population is large and difficult to study as a whole. To help with this problem we use statistics. An statistic refers a value that states something about a sample of a population. A sample is a portion of the population we are studying. For example, we want to know the average age of the population of a city. We can ask for the ages of every single citizen, but this would consume a lot of time and resources. Instead, we pick a sample of a certain number of people, ask each one for their ages and calculate the mean. This mean would be an statistic used to aproximate the parameter of the total population. The bigger the sample, the more representative will be the statistic.
With this in mind, the average value calculated in the problem is the mean of a sample of 24 days. This means that value is a statistic. In order for it to be a parameter you would have had to measure the voltage of every single day and calculate the mean with this information.
