How do I create user-friendly drop-down lists where you can select multiple options?

I know how to create drop-down lists, and how to create data validation drop-down lists where you can choose multiple answers. My issue is, I can't figure out how to create said lists where, if you accidentally choose a second answer, or the wrong second answer, it's possible to erase that second answer. Right now when I select a second (or third, or fourth, etc.) answer, I'm not able to delete it to go back to just having the single answer.
So...
if the drop-down list options are "a, b, c, and d," and I first choose "a," I currently have it set up so that I can then click back on the list, and choose another option so that I now have "a, b" in the cell (or a, d or a, c, etc). I can't delete "b" then, though, so I'm stuck with "a, b" in that cell.
If I'm going to sell this to my coworkers as something they can use as a data tracking tool, it's got to be very user-friendly. Is there any way to get this to happen in a way that wouldn't require multiple steps and would just be as simple as deleting that second answer?
Thanks much to anyone who can help assist this ignorant unpaid intern!

Ok, here's an example doc. The cell I'm looking at is obviously A1. This is the code I'm working with, which allows me to select multiple choices, but does not allow me to delete any selections, which is why the cell block is currently stuck with three choices, "C, d, b"

Thanks for the feedback! I guess what I'm trying to do is what you pointed out in your last statement: I'm trying to make it so you can remove a single value from the list. Would you have any recommendation on how to edit my code in order to do that? I'm kind of lost in the woods on this. I'm beginning to think it's not possible but if any easy suggestion jumps out at you please let me know. Thanks again!

Interesting. Thanks!

I'll take a look and see if I can make this work. Thanks!