Why might a balance of power prevent war

when balance of power exist between two nations, all the countries that are involved in a conflict would be fully aware that each of them had the capability to create massive destruction toward one another.

Because of this awareness, both parties would become hesitant in making initiation to start a war and might choose diplomatic method to solve their difference.

Nobody will feel the need to start war.

Explanation:

Sometimes one will start war as an act of self defenses, if everybody is equal there is no reason to feel threatened by another.

i think it's a)sparta

powers  are  denied  to the  national governmentin three distinct ways: some  powers, such as the  power  to levy duties on exports or prohibit the freedom of religion, speech, press, or assembly, are expressly  denied  to the  national government  in the  constitution.