Answer:
D. The GDP deflator but not the CPI
Explanation:
GDP deflator is a tool used in measuring the changes in prices of goods and services produced in the country. It is measured by dividing nominal GDP with the real GDP. Since the Gun is part of the GDP, hike in prices will affect the GDP deflator.
On the other hand Consumper price index measures changes in prices of basket of goods and services consumed by households. The guns are not been bought by households but rather they are bought by the army in the US. Therefore changes in the price doesn't affect CPI.