Let's differentiate the terms statistic and parameter. Statistic is a branch of mathematics that involves the treatment of various data through collection, preparation, analysis and interpretation. A parameter is a unit of measure which is an element of the system.
Since 78% is the percentage of the whole data, this is a statistic. A parameter, for instance, is the 2,890 readers :)
The answer is A.
I hope you clearly understood my explanation :)
