The correct answer is: "Both believed in the theory of a social contract."
Rousseau and Locke are two very important philosophers from the Enlightment era. The Enligthment, in general, consisted on the promotion of reason and of scientific methods over previously dominant superstitions, dogmas and religious beliefs.
The social contract theory states that the power of a state resides on its citizens, who have the right of electing their political representatives through suffrage. This was a quite revolutionary idea, as it emerged in Europe when most countries were governed by absolute monarchs whose power was believed to be granted by God.