Explanation:

- In the Keynesian model, the aggregate expenditure (AE) is often depicted on a graph. This model visually represents how income levels are determined by the total spending in the economy.

- Option 1: Vertical axis - This is the correct choice. The aggregate expenditure is measured along the vertical axis in the Keynesian cross diagram.

- Option 2: Horizontal axis - The horizontal axis typically represents real GDP or income.

- Option 3: Vertical intercept - This refers to where a line crosses the vertical axis, not a measurement axis for AE.

- Option 4: 45° line - This line represents points where aggregate expenditure equals total output or income, showing potential equilibrium.