In macroeconomics, when a price is constant, equilibrium is achieved at a point where?

Ex ante Aggregate Demand = Ex ante Aggregate Supply.

The correct answer is Option (3) → Ex ante Aggregate Demand = Ex ante Aggregate Supply.

• Ex ante Aggregate Demand includes planned consumption + planned investment 

• Ex ante Aggregate Supply refers to the planned output (income) that producers intend to supply.

Equilibrium is reached when the planned expenditure by all sectors of the economy matches the planned production/output, i.e., $$\text{Ex ante AS}$$

When the total amount of goods and services that everyone in the economy plans to buy (ex ante aggregate demand) is exactly equal to the total amount of goods and services that firms plan to produce (ex ante aggregate supply), there are no unintended inventory changes, and thus no incentive for firms to adjust their production levels. This leads to a stable equilibrium level of output and income.