Final answer:
The horizontal axis of the aggregate demand-aggregate supply model measures Gross Domestic Product (GDP), which represents the quantity of output or economic activity in the economy.
Step-by-step explanation:
The horizontal axis of the aggregate demand-aggregate supply model measures Gross Domestic Product (GDP), which is the sum of all final goods and services produced in the economy. It represents the quantity of output or economic activity in the economy. For example, if the horizontal axis measures GDP in billions of dollars, a point on the axis at 200 would indicate a GDP of 200 billion dollars.