The correct answer is: D. Turn power off before removing power source.
The high-voltage anode capacitor in a CRT can store a dangerous charge even after the power has been turned off. To discharge the capacitor, you must first turn off the power and then disconnect the power cord from the outlet. Once the power is disconnected, you can safely remove the CRT from its housing.
Option A is incorrect because it is not safe to remove the CRT from its housing until the power has been turned off and the capacitor has been discharged.
Option B is incorrect because it is not necessary to disconnect the CRT from the computer before turning off the power.
Option C is incorrect because it is not necessary to remove the video assembly before turning off the power.
Option E is incorrect because it is necessary to turn off the power before removing the power source.