The president doesn't like the idea of a national health care plan, but she
supports the plan because most citizens prefer it. Which constitutional
principle does this scenario best illustrate?

The correct answer is Popular sovereignty

Sovereignty, in its political or legal sense, is the exercise of authority that resides in a people and that is exercised through its representative constitutional organs. Sovereignty is a superior authority that cannot be restricted by any other power and, therefore, constitutes itself as the absolute power of legitimate action in the political and legal sphere of a society.

A state in which Popular Sovereignty rules is created and subject to the will of the people, who are the source of all political power. It is the basic principle of democracies.

what the first or next guy says

b.   jim crow laws were enacted in northern states.