The correct answer is A. Robbins.
Lionel Robbins was a British economist who is considered one of the founders of modern economics. He is best known for his book “An Essay on the Nature and Significance of Economic Science” (1932), in which he defined economics as “the science of human behavior as a relationship between ends and scarce means which have alternative uses.”
Robbins’ definition of economics has been widely influential, and it is still used today. It emphasizes the fact that economics is a science of choice, and that resources are scarce. This means that people have to make choices about how to use their resources, and that these choices will have an impact on their welfare.
Marshall, Ricardo, and Keynes were all important economists, but they did not define economics in the same way as Robbins. Marshall defined economics as “the study of mankind in the ordinary business of life; it examines that part of individual and social action which is most closely connected with the attainment and with the use of the material requisites of well-being.” Ricardo defined economics as “the science of the production and distribution of wealth.” Keynes defined economics as “the science which studies human behavior as a relationship between ends and scarce means which have alternative uses.”
All of these definitions are useful, but they each emphasize different aspects of economics. Robbins’ definition is the most general, and it is the one that is most commonly used today.