Answer:
E. Check the simple things first to make sure you do not miss anything
Explanation:
This first step is called troubleshooting. As a certified operator, you need to check the simple things first to make sure you don't overlook something in your diagnosis.
The client is fairly certain the problem is from the video card, so after checking the simple things first (troubleshooting) and see no problems, then you can check the video card of the computer.
The reason it is advisable to troubleshoot on the simple things is to make sure you don't overlook something and also that your mind isn't totally fixed on one particular thing which is the video card in this situation.