The correct answer is B) try to prevent domination by one country.
When nations try to maintain the balance of power they try to prevent domination by one country.
After two World Wars and the constant result of expansionism, influences and the exercise of power between countries, powerful nations try to maintain the balance of power they try to prevent domination by one country. History has indicated that when one country try to dominate another country and has expansionist goals and starts to take measures to influence other countries, there is no balance of power and these actions can generate instability and conflicts in the region. That is why it is so important to avoid the domination of one country over the others, maintaining the balance of power.
The other options of the question were a) promote equality among social classes, c) try to reduce their armies, and d) promote peace among nations.