The correct answer is: C. Alfred Marshall
The law of diminishing returns states that in all productive processes, adding more of one input, while holding all others constant, will at some point yield lower incremental per-unit returns.
Alfred Marshall was an English economist who is considered one of the most influential economists of the 19th century. He is best known for his work on microeconomics, including his development of the theory of marginal utility. Marshall also made significant contributions to the field of macroeconomics, including his work on the theory of money and prices.
Walras, Pigou, and Ricardo were also economists, but they did not make the same contributions to the law of diminishing returns as Marshall. Walras was a Swiss economist who is considered one of the founders of neoclassical economics. Pigou was an English economist who is best known for his work on welfare economics. Ricardo was an English economist who is best known for his work on classical economics.
In conclusion, the statement that the law of diminishing returns is applied to all fields of production was stated by Alfred Marshall.