Of the cases that you have mentioned in this question, the one that best fits the question is C.
Mapp v. Ohio was a landmark case in applying the 4th Amendment to the states and localities. It was a much broader statement of principle than more technical cases like Horton or Gates. In Mapp, the Court ruled that the exclusionary rule (which comes out of the 4th Amendment and others) applied not only to the federal government but also to state governments. Since local governments are simply subsets of state governments, the case applied to them as well. The other cases mentioned merely addressed details of exactly how the 4th Amendment applies to the states (or, in Leon, to the federal government).