The correct answer is C) inject fuel at the proper time. Fuel injection systems are designed to primarily meter the correct amount of fuel, atomize the fuel into a fine mist, and inject the fuel into the combustion chamber at the proper time to ensure efficient combustion. Injecting the fuel at the proper time is a critical function of the fuel injection system, as it allows the fuel to be introduced into the cylinder at the optimal point in the engine's cycle for complete and efficient combustion. The other answer choices are incorrect because they do not directly address the primary purpose of a fuel injection system, which is to meter, atomize, and time the fuel injection. Creating turbulence in the combustion chamber (A), aiding in cylinder scavenging (B), and minimizing fuel penetration (D) are secondary effects or functions that may be achieved by the fuel injection system, but they are not the primary objectives.