How to make choices in a list box hidden when another choice is selected?

[Julian Beirne]

I am trying to make a labourous paper form in my company into a slick pdf digital form.
The form has a lot of boxes with long answer choices.
I need the form filler to select one, or multiple choices and then when s/he clicks on the next box down the form the other options on the first box are hidden.
I have got so far reading the guides on here and yoootube but have reached my understanding limit.
This is the Word form. 

The form filler will select one or many different options then move on down to the next box. (or even none, in which case they must all be hidden)
I've tried the .hidden and .visable option but look at the text length!
My pdf form is 'ugly' I am told. 😄 Look at the length of the answer in OP-S-PS! 
Auto Text size makes them all differenet sizes.

Now this is my Form attempt.

I have selected the top option but the rest are still visible. 

I don't know javascript but knew Vic basic and BBC Basic back in the 80's so can understand what javascript is doing.
BTW I think this digital form stuff is brilliant! Real 21st Century stuff. 
Thanks for your help.

[Karl Heinz Kremer]

A listbox (or a dropbox) are very simple controls compared to what you can do with e.g. the controls that MS Windows or macOS provide. Unfortunately, there is no mechanism to disable certain elements from a listbox or dropbox. You would have to populate the control with a new set of options in order to remove choices. You can keep a master list that contains everything, and then based on the current form state, you would filter that and remove options that are not appropriate. When the form state changes again, you would update that control again with another set of options.