According to the monetarist theory, a critical causal force that determines the nominal gdp..

money supply

Explanation:

Monetarists are a branch of new classical economists that, as the name  suggests, believe that money has a very important part to play within an  economy.They believe that aggregate expenditures in the economy are influenced by the  market rate of interest, and therefore money can affect the level of output in the  short run economy.However, they further believe that money influences the long run unemployment  in the economy. If monetary policies are used to increase aggregate demand, it is  thought that this use of additional money may cause a short term boost in output,  but will ultimately lead to inflation in the economy.

So the answer is money supply

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