Should dropdown list option be removed if the options cannot be repeatedly selected?

Question

Let's say, I have a dropdown (html select) of questions from which a teacher can pick up questions to present to their students. The options available in the dropdown can only be picked once i.e. the teacher cannot pick up same question twice.

So, now my question is:

If the options cannot be picked more than once,

A) should the dropdown option list should be updated to show only the unpicked options ?

OR

B) the dropdown should be left untouched, and the selection should not be updated if the user clicks on an already picked-up option ?

OR

C) gray-out(disable) the already picked-up options in the dropdown list ?

Which one would be the better and more intuitive approach?

screenshot of our current UI:

Answer 1

A drop down is probably not your best option here. To make this task clearly understandable a more suitable UI would be some form of dual list as in the example below: This does not only offer a selection mechanism but also a constant visibility of what has been selected.

Update:

Given that you have limited manoeuvre around the design i would suggest the following:

A) should the dropdown option list should be updated to show only the unpicked options ?

Don't think this is a good idea as it removes control from the user and does not offer visibility in terms what has been selected.

OR

B) the dropdown should be left untouched, and the selection should not be updated if the user clicks on an already picked-up option ?

This create a problem as it breaks the feedback loop Action > Effect > Feedback.

OR

C) gray-out(disable) the already picked-up options in the dropdown list ?

Given design limitations I would recommend this option as suggested by @JonW as it covers the usability downsides from both option A & B

Here is an example of how it might look:

Hope this was of help

Answer 2

I was inspired recently by @jeffatwood 's excellent article about The God Login where one would ask:

What would God do?

Well if God was doing this, there wouldn't even be a selection list - the option the user wants would be right there already - no selection necessary. God already knows which one you want.

The drop down is being used for someone to pick a bunch of questions from a drop down and add them to a list. God wouldn't need a drop down. God would just give you a list of questions and as Jeff says: Bam. One Step.

OK - but say the teacher does want a bit of control over what goes in the list, and the drop down is needed. Well they want to be able to see which ones they've added, sure (like @JonW said) - but they're more interested in the one's they haven't added yet. They can already see which one's they've added because they're right there in the list of questions they're building.

Let's imagine the worst case - 100 questions in the drop down. The teacher sequentially adds number 1,2,3,4,...99. Starts off well, but by about item 60 the teacher has to scroll way down the list to get the next one.

Well God would say you should only need to look at the list once and select everything in one go. Bam! So that means some way of keeping the list open and making multiple selections - like a multi-check list.

But what if the order mattered and it didn't match the order in the list. Well just add the questions in the order that the user clicked on them.

But what if the teacher did want to do it in more than one step. Then the list needs to allow multiple selections and show which ones have been added already.

But what if the teacher wanted to be able to see the order in which they're making a selection - while the list is still up. So maybe the list slowly fades the colour of an item in or out as the items get selected so the items age. And not only that, God would let you shuffle items around in the final list of questions too.

God still wants you not to use a drop down list so maybe God combines the functionality of the list of final questions and the above functionality of the drop down list into a single component where you can promote, demote, shuffle, and randomize one or more (or all) items at once.

What I'm saying is - you choose your priorities and think about the absolute best possible outcome - the path of least resistance - the option that seems like the system is reading your mind - the way God would do it. And see how close you can get.

Hell yeah - The God Login is the way we should be thinking!