Based on choices I've seen elsewhere for this question, the answer is likely: Observing traffic laws in exchange for driving on the highway.
Traffic laws are established by the government, and roads and highways are built by governments (local, state and federal). The "social contract" refers to an implicit agreement between a government and the citizens of the society overseen by that government. Examples that have to do with business exchanges--such as paying an electric bill in exchange for electricity usage--are examples of business contracts rather than "the social contract."
Philosophers of the Enlightenment era were famous for arguing the idea of a "social contract." According to this view, a government's power to govern comes from the consent of the people themselves -- those who are to be governed. This was a change from the previous ideas of "divine right monarchy" -- that a king ruled because God appointed him to be the ruler. One of the most influential of the social contract theorists was John Locke, who repudiated the views of divine right monarchy in his First Treatise on Civil Government. In his Second Treatise on Civil Government, Locke then argued for the rights of the people to create their own governments according to their own desires and for the sake of protecting and enhancing their own life, liberty, and property.