Explanation:

- Option 1: Wage price rigidity

- The Keynesian perspective suggests that wages and prices are "sticky" and do not adjust quickly to changes in demand.

- This rigidity leads to a perfectly elastic aggregate supply curve at low levels of output.

- Option 2: Constant Marginal Product of Labour

- Assumes that the marginal product of labor remains constant, implying that additional labor can be employed without increasing wages.

- Contributes to the horizontal nature of the Keynesian aggregate supply curve.

- Option 3: Both of these

- The combination of wage price rigidity and constant marginal product of labor supports the perfectly elastic supply curve in Keynesian analysis.

- Option 4: None of these

- If neither factor was applicable, this would be correct, but it does not apply here.

The correct answer is Option 3: Both of these.