Real interest rate equals nominal minus expected inflation is:

The quantity theory of money posits a direct relationship between the money supply and price levels in an economy, typically expressed as MV = PQ. While it addresses inflation, it does not directly relate to the calculation of real interest rates, making it an incorrect choice for this specific question.

The money multiplier refers to the maximum amount of money that banks can create with each dollar of reserves, influencing the overall money supply. Although it plays a role in monetary policy, it does not involve the relationship between nominal interest rates and inflation, rendering it irrelevant to the question.

The Keynesian cross model illustrates the relationship between aggregate demand and total output in an economy, focusing on equilibrium levels of income. It does not pertain to interest rates or inflation directly, making it an unsuitable choice for defining the relationship asked in the question.