If you just need to cancel the 3-columns layout for an open question at the end, you can use something like

*([last] [[ \end{multicols} ]]

*<lines=5>
+ Correct
- Wrong

*) [[ \begin{multicols}{3} ]]

Thank you for your suggestion.
I should emphasize that I am working in AMC-TXT. Latex is enabled and multicol package loaded but I get the
following string of error messages when I try to append the code above.

8 errors during LaTeX compiling (question sheet)
LaTeX Error: \begin{document} ended by \end{multicols}.
Missing } inserted.
Extra \endgroup.
LaTeX Error: \begin{document} ended by \end{multicols}.
Extra \endgroup.
Extra }, or forgotten \endgroup.
LaTeX Error: \begin{multicols} on input line 504 ended by \end{document}.
Package multicol Error: Document end inside multicols environment.

Can you send your source file?

Sure, here it is

I see: my trick only works without separate answer sheet, sorry...

Thank you for your assistance, I guess I will add the lines to the pdf file
using some pdf editor.

Best,

Niko Z.

This is just to say that I am facing the same problem but I couldn't find a solution.
I have exactly the same exam structure with a separate answer sheet and I want to add
an open question at the end.

Well, sorry, I just found an easy solution...
I just a pice of code to generate the rows directly into the question.
You can find the code easily by searching for "table to fill page exam.cls" on Google.

Lello

Interesting, this is certainly helpful.
The added code blends well with your template?
No problems during grading?

Thanks,

Niko Z.

Yes, the code blends perfectly, but I have not yet checked if it works fine when grading but.
Technically, there should be no problem, I add the questions after the answer sheet, it only helps
me to make photocopies and save a copy of all the exams.