The correct answer is D.
A D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor.
This is because the motor will not be able to handle the higher ambient temperature and will overheat. De-rating the motor will reduce its power output and allow it to operate safely in the higher ambient temperature.
Option A is incorrect because a motor of lower H.P. will not be able to handle the load at the higher ambient temperature.
Option B is incorrect because a motor of higher H.P. will not be necessary if the motor is de-rated.
Option C is incorrect because the motor will not be able to handle the higher ambient temperature without de-rating.