What is the opportunity cost (in civilian output) of a defense buildup that raises military spending from 4.1 to 4.5 percent of a $7 trillion economy?

Assuming the build-up in military defense completely crowds out civilian output (as resources are diverted from the private sector), then the 0.4% increase in military spending will lead to an equal 0.4% decrease in civilian output (as a proportion of the $7 trillion economies). 
You need to multiply $7 Trillion by 0.004 (0.4%) 
$7,000,000,000,000 X 0.004 = $28,000,000,000 
and the opportunity cost (in civilian output) of a defense buildup is $28 Billion.
Opportunity cost is the benefit that the person can be received, but they gave up, to take another course of action. It is also a benefit, profit or value of something they give up to achieve or acquire something else.

The first answer is b. then the second one is a