Even economists who advocate small government agree that
a. national defense is a common resource and that the government should provide it.
b. national defense is a common resource, but many of them believe that it should be provided by private firms rather than by the government.
c. national defense is a public good, but many of them believe that it should be provided by private firms rather than by the government.
d. national defense is a public good and that the government should provide it.

D

Explanation:

Only anarcho-capitalists believe that defense should be provided by private firms.

A more general approach is that there are primary and secondary functions in the state.

Primary functions are defense, legislation, judiciary, and so on. Basically this means that the state should provide public goods

Secondary functions might include non-public goods such as education, health, energy and so on.

A good rule of thumb is that when economist say "small" goverment they mean that it should only take care of it's primary functions