The correct answer is: B. Marshall.
Alfred Marshall was an English economist who is considered one of the most important figures in the development of neoclassical economics. He is best known for his work on the theory of value, which he developed in his book Principles of Economics. In this book, Marshall introduced the concept of equilibrium firm, which is a firm that is producing at a level where its marginal revenue equals its marginal cost.
Pigou, Robbins, and Mehta were all economists who made significant contributions to the field, but they did not develop the concept of equilibrium firm. Pigou was a British economist who is best known for his work on welfare economics. Robbins was a British economist who is best known for his work on the theory of choice. Mehta was an Indian economist who is best known for his work on the theory of economic development.
The concept of equilibrium firm is important because it provides a framework for understanding how firms make decisions about production and pricing. It also helps to explain how firms interact with each other in the market.