Since updating to the latest version of the V5 Branch (5.6.44) the code hack that Holch referred to no longer works as the files have been updated.
Like others, I never create surveys without mandatory questions, so having this as a variable that we could set somewhere would be super useful.
After doing a little digging around I have found that there is a single file that needs to be adjusted to make it happen.
WARNING: Its Still a code hack - so be careful and make sure you update regularly and replace the hack if it gets overwritten.
Edit the file: /application/models/QuestionCreate.php
As of 5.6.44 the line you want is Line 43. (might change in later versions)
Change:
'mandatory' => 'N',
to
'mandatory' => 'Y',
Done!
18/11/23 Quick update to this - the instructions are Valid as of Version 6.3.5 as well