Explanation:

Bipolarity (answer a) is a situation in which two powers are dominant (e.g. during the Cold War). A balance of power (answer b) might arise from the distribution of power among three or more states, but it is not inevitable in such circumstances. Equally, such a situation might resemble Thomas Hobbes' state of nature, but it is possible to characterize any international system as a state of nature because there is no supra-national 'sovereign' (answer c)