Answer: b. popular sovereignty
Step-by-step explanation:
The democratic ideal of Popular Sovereignty is one that is rooted at the very core of democracy. It stipulates that the authority that the Government has, is granted and sustained by the people of that nation when they vote for that Government.
In the graph, we see that the President was voted into power by people of the United States and that even though the proportion of the people who did so out of the total population was less in the past, the President was still only the president because the people voted them in. This therefore reflects Popular Sovereignty.