Answer:
D) Decrease by $50 billion
Step-by-step explanation:
If the marginal propensity to consume (MPC) = 0.8, that means that the marginal propensity to save (MPS) = 1 - MPC = 1 - 0.8 = 0.2
If investment spending falls by $10 billion, then aggregate demand will decrease by: $10 billion / 0.2 = $50 billion.