Final answer:
Socrates considered justice to be the fundamental principle on which all human society should be based, highlighting its role in achieving a harmonious and successful community.
Step-by-step explanation:
According to Socrates, the fundamental principle on which all human society should be based is justice. Socrates, along with his student Plato, emphasized the role of justice, wisdom, courage, and temperance as the four virtues necessary for a well-ordered society. However, justice stands out as the key virtue for Socrates, particularly in Plato's Republic, where he uses the analogy of the just city to illustrate how a society based on justice would function harmoniously, providing avenues to happiness and success for individuals and the community as a whole. Hence, the answer to the student's question is C. Justice.