The correct answer is B) It protects citizens' rights against government mistreatment.
The Bill of Rights was added to the Constitution after much debate between Federalists and Anti-Federalists at the Constitutional Convention. The Anti-Federalists felt the Bill of Rights was a necessity, as there needed to be a clear set of rights that citizens have that cannot be taken away by the government.
A perfect example of this is the due process clause. This ensures that people accused of comitting crimes have certain rights (i.e. right to a jury trial, right to an attorney).