The correct answer is D. One.
An outlet is said to be proportional if its flexibility is one. This means that the outlet will always provide the same amount of power to the device that is plugged into it, regardless of the voltage or current of the device.
Option A is incorrect because an outlet with zero flexibility would not provide any power to the device that is plugged into it.
Option B is incorrect because an outlet with flexibility less than one would provide less power to the device that is plugged into it than the device requires.
Option C is incorrect because an outlet with flexibility greater than one would provide more power to the device that is plugged into it than the device requires. This could damage the device.