The SI unit of resistivity, which is the measure of how strongly a material resists the flow of electric current, is represented by the option 4, "Q m."

Option 1, "Q," is incorrect as it represents coulombs, which is the unit of electric charge, not resistivity.

Option 2, "Q/m," is incorrect as it also represents coulombs per meter, which is the unit of electric charge density, not resistivity.

Option 3, "Ω cm," is incorrect as it represents ohm centimeters, which is the unit of electrical resistivity in the CGS system, but not in the SI system.

Therefore, the correct answer is option 4, "Q m," which represents the SI unit of resistivity, measured in coulombs per meter.