Explanation:

- If an economy tries to save all its extra income, that means people are not spending, so consumption drops.

- In the simple Keynesian model, if everyone saves all extra income, consumption becomes zero.

- Since investment (in national income equilibrium) depends on savings being spent, no spending means firms have no incentive to invest.

- This results in actual investment being zero.

- Let's look at the options:

- Option 1: 1 – Implies investment remains 1. This is incorrect.

- Option 2: Uncertain – Investment is not uncertain; it can be calculated.

- Option 3: 0 – Correct. Investment will be zero because there's no demand.

- Option 4: Infinite – Investment cannot be infinite in this scenario.