The development of new nations is often difficult because
a. governments are often unstable because of civil wars and other conflicts
b. increasing populations deplete the resources available
c. they are economically dependent on other nations
d. all of the above

I don't want to be that guy, but D. All of the above sounds logical. Usually new nations for because of a civil war and/or a major conflict. For example, in 2014 I believe, when Ukraine was in a big riot, they gave up a bit of land to Russia. Increasing populations do make it harder for resources to be available. If you've heard anything about North Korea, two-thirds of the country is practically in a chronic starving condition. Also, suburbs in the United States take up quite a majority of space, and many of those suburbs used to be farmlands. New countries are probably going to be economically dependent as they are fresh and trying to figure out things. This will lead to a fear of large debt they may not be able to pay off.

The answer is ethnic groups.