You will need 2 listings - one for the combination of customer D and product X and another for all other products except product X on the level of customer group for example. You set the procedure as 'B' - at least one listing should apply.
For customer A you will get a listing of all products except X. The first listing cannot be determined because it is a different customer.
For customer D you get a listing for all products as for customer A and the listing containing product X.
Make a quick test in your development system with the settings proposed and I am sure that you will understand it easily once you try it with 2-3 customers and materials. Image may be NSFW.
Clik here to view.
Another approach is to use the exclusion functionality and forbid the sales for product X to all customers except D, but it can be risky, because when somebody creates a new customer he can forget to update the exclusion list. To minimize the problem you can create the exclusion based on one of the freely definable fields for customers (like KATR*, KDKG*, KVGR*), that you set as obligatory for the account group.
Also: if you do not have listing or exclusion maintained for a customer, you can sell all products to him and you do not want that to happen. So either create exclusion based on customer attribiues that can be activated even for new customers or create the listings in such way as to get the second listing applied for a new account as well.
