The distinguishing feature of microeconomics is studying: a

The correct answer is b) Choices made by individuals or groups in the context of individual markets.

This answer is quite straightforward from the very definition of microeconomics, which states that microeconomics is concerned about the study of individual choices, individual markets, partial equilibrium, etc.

Moreover, the topics stated in a), c) and d) are concerns of macroeconomics, not microeconomics (macroeconomics deal with the study of economics phenomena at aggregate and national level).

Finally, e) is a general statement that concerns to economics in general, not only microeconomics or macroeconomics, but any field of study