The Incorporation Doctrine primarily serves to prevent states from restricting a citizen’s civil rights. Through the doctrine, the protections guaranteed by the Bill of Rights have been interpreted to apply to state governments via the Fourteenth Amendment's Due Process Clause. This means that states cannot infringe upon the rights that are protected by the federal Constitution, thus increasing the power of the federal government by ensuring that individual rights are upheld uniformly across the country, regardless of state laws.
In summary, the correct answer is:
It prevents states from restricting a citizen’s civil rights.