The study of a single firm and how it determines prices would fall under:

It will fall under microeconomics. Microeconomics, from the prefix micro meaning small, it is a branch of economics that studies the actions of individuals and firms in making choices regarding the distribution of limited resources and the relations among these individuals and firms. One goal of microeconomics is to examine the market mechanisms that establish comparative prices among goods and services and allot limited resources among alternative uses. Microeconomics shows situations under which free markets lead to required allocations. It also analyzes market failure, where markets fail to make effective results.