Option 4: 4 -> The investment multiplier is calculated using the formula: k = 1/(1-MPC). From the given aggregate demand function AD = 80 + 0.75Y, we can identify that the marginal propensity to consume (MPC) = 0.75. Therefore, the multiplier k = 1/(1-0.75) = 1/0.25 = 4. This means that a one-unit increase in investment will lead to a 4-unit increase in national income through the multiplier effect. -> correct