The correct answer to the question about the impact of a $1 billion increase in investment on GDP can be found using the concept of the multiplier effect in economics. The multiplier is calculated as:
\[ \text{Multiplier} = \frac{1}{\text{MPS}} \]
Where MPS = Marginal Propensity to Save.
Thus, a $1 billion increase in investment will cause a multiplier effect that leads to an increase in GDP of:
\[ \text{Increase in GDP} = \text{Investment} \times \text{Multiplier} = 1 \text{ billion} \times \frac{1}{\text{MPS}} \text{ billion} \]
So the correct answer is:
(1 ÷ MPS) billion increase in GDP.