Since we're talking about nations, it is likely that the answer itself will also have the word "nations" in it: so let's look at the options c. and d.
They are only in early states, and it is assumed that they will develop further.
Therefore the correct answer is "c., developing nations".
The answer is in the 'necessary and proper clause' of the U.S. Constitution, better known as the 'elastic clause,' which allows Congress to make laws it needs to carry out its own powers.
Hope this helps. =^D