The correct answer is: A. Price theory.
Microeconomics is the study of the behavior of individual economic agents and how they interact with each other in markets. It is concerned with the allocation of scarce resources among competing uses. Price theory is the branch of microeconomics that studies the determination of prices in markets. It is based on the assumption that individuals are rational and will make choices that maximize their utility.
Product theory is the branch of microeconomics that studies the behavior of firms and how they produce goods and services. It is concerned with the costs of production, the choice of inputs, and the output of firms.
Process theory is a branch of economics that studies the way in which economic systems change over time. It is concerned with the factors that cause economic growth, the causes of economic crises, and the effects of economic policies.
Project theory is a branch of economics that studies the planning and evaluation of investment projects. It is concerned with the identification of project opportunities, the estimation of project costs and benefits, and the selection of the best projects to undertake.