The correct answer is (a) Knapp.
Georg Friedrich Knapp was a German economist who is best known for his book “The State Theory of Money” (1905). In this book, Knapp argued that money is a creature of the state, and that its value is derived from its use as a unit of account and a medium of exchange. Knapp’s definition of money is often summarized as “Anything which is declared money by state becomes money.”
Ely was an American economist who is best known for his work on labor economics. Keynes was a British economist who is best known for his work on macroeconomics. Seligma was an American economist who is best known for his work on monetary policy.
Ely, Keynes, and Seligma all had different definitions of money. Ely defined money as “anything that is generally accepted as a medium of exchange, a measure of value, and a store of value.” Keynes defined money as “the aggregate of the means of payment in the possession of the public.” Seligma defined money as “the stock of currency and demand deposits in the hands of the public.”
Knapp’s definition of money is different from the definitions of Ely, Keynes, and Seligma in that it emphasizes the role of the state in creating money. Knapp argued that money is a creature of the state, and that its value is derived from its use as a unit of account and a medium of exchange. Ely, Keynes, and Seligma did not emphasize the role of the state in their definitions of money.