Constraint for Multipale Choice

Dear All,
Hope every body doing well! I have a question for constraint in multiple question I would appreciate if any one could support me, in a details I attached the screen shot for my question to make a better sense.
Thanks,
Samim

Samim,

You should be able to do this with a validation condition. I think this should do it:

if(selected(#form/why_not_treat, '999'), #form/why_not_treat = '999', true())

That says:

if ‘999’ is selected, the answer must be exactly ‘999’ (no other selections)
if ‘999’ isn’t selected, anything is fine.

Dear Ethan_Soergel,
Thanks for collaboration! yest it’s working well!