The correct answer is - E.) to increase national sovereignty.
By cooperation on regional or global level the states do not, and can not really do anything about increasing the national sovereignty. The reason for that is that the national sovereignty is the matter of the state itself, its politics, policies, and handling of the situations in the country. The other countries with whom the state cooperates have nothing to do with it, nor it is their place to interfere in the national sovereignty of the other states, be it friendly states or not.
