The correct answer is C) higher torque and higher brake horsepower. Aftercooling of a turbocharged diesel engine results in increased air density entering the engine cylinders. This higher air density allows for more fuel to be injected, leading to higher combustion pressures and temperatures. The increased combustion efficiency translates to both higher torque and higher brake horsepower output from the engine. The other options are incorrect because: A) and B) are not accurate, as aftercooling improves both torque and horsepower. D) is incorrect, as the increased air density and combustion efficiency lead to higher, not lower, torque and horsepower.