Answer: The answer is $ 1 billion.
Explanation:
MPC stands for the marginal propensity to consume.
If MPC is 9 it implies that the multiplier is 10 i.e 1/(1-0.9). The rise in aggregate demand is equal to multiplier times change in government expenditures so to boost aggregate demand by 10 billion dollar government has to increase expenditure by Dollar 1 billion.