Correct Option: A

Macroeconomics is a branch of economics in which a variety of economy-wide phenomena is thoroughly examined such as, inflation, price levels, rate of growth, national income, gross domestic product and changes in unemployment. On the other hand, Microeconomics studies the behavior of individuals and firms in making decisions regarding the allocation of scarce resources and the interactions among these individuals and firms. So the study of sales of mobile phones by BSNL comes under microeconomics.