Among the fundamental concepts in economics is

A) opportunity cost.
B) marginalism.
C) efficient markets.
D) all of the above

All of the options listed - opportunity cost, marginalism, and efficient markets - are fundamental concepts in economics. Opportunity cost refers to the cost of the next best alternative when a decision is made; marginalism focuses on the additional or incremental changes in economic variables; and the efficient market hypothesis suggests that financial markets are "informationally efficient," meaning that asset prices fully reflect all available information.