The correct answer is C. Gustav Cassel.
Gustav Cassel was a Swedish economist who is considered one of the founders of modern international economics. He is best known for his work on purchasing power parity, which is the idea that the exchange rate between two currencies should be equal to the ratio of their purchasing powers in the two countries. Cassel developed this theory in the early 20th century, and it came into prominence in 1916 through his writings.
The other options are incorrect. A. J. M. Keynes was a British economist who is considered one of the most influential economists of the 20th century. He is best known for his work on macroeconomics, and he developed the theory of general equilibrium. L. E. Von Mises was an Austrian economist who is considered one of the founders of the Austrian school of economics. He is best known for his work on monetary theory and capital theory. F. A. von Hayek was an Austrian economist who is considered one of the founders of the Austrian school of economics. He is best known for his work on monetary theory, business cycles, and socialism.