The correct answer is C) higher compression pressures. Diesel engines generally have higher compression ratios compared to gasoline engines, which results in higher compression pressures. This higher compression pressure requires a more powerful starting motor to overcome the resistance and crank the engine. The diesel engine starting motor needs to provide sufficient torque to turn over the engine against these higher compression pressures. This is why diesel engine starting motors are designed with heavier duty components and operate at higher voltages than their gasoline engine counterparts. The other answer choices are incorrect because: A) Higher speed is not the primary reason, as the starting motor speed is determined by the engine requirements. B) Lower starting temperatures do not directly necessitate higher voltage starting motors. D) The flywheel effect is more related to the engine design, not the starting motor requirements.