Important political thinkers:
• Rousseau $\rightarrow$ The Social Contract
• Montesquieu $\rightarrow$ The Spirit of Laws
• John Locke $\rightarrow$ Two Treatises of Government
Concept:
The Social Contract is a famous political philosophy book written by the French philosopher Jean-Jacques Rousseau in 1762.
Step 1: Identify the author.
Rousseau explained that legitimate political authority is based on the general will of the people and the idea of a social contract between citizens and the state.
Step 2: Analyse the options.
• Montesquieu -- Wrote The Spirit of Laws.
• Voltaire -- Famous French Enlightenment writer and philosopher.
• John Locke -- Wrote Two Treatises of Government.
• Rousseau -- Wrote The Social Contract. \checkmark
Step 3: Final conclusion.
Hence, the author of The Social Contract is
\[
\boxed{\text{Jean-Jacques Rousseau}.}
\]