Choice "B" is correct. Power concentrated in a single nation is referred to as a unipolar distribution of power.
Choice "a" is incorrect. Action taken by a single nation acting on its own is referred to as unilateral. The distribution of power is a separate issue.
Choice "d" is incorrect. Power distributed amongst multiple nations refers to a multipolar distribution of power.
Choice "c" is incorrect. Multilateral refers to power that is not only distributed but shared cooperatively among nations.
