you can’t of course use the ‘mandatory answer’ on the table as that will only check to see that table HAS been answered and won’t know what was done in each Row
but running a behaviour on the appropriate event (maybe in your case the ‘table row has lost focus’) that then checks to see at least one of the check boxes is checked, and if none are then ‘show message’ to user may do what you need
Alternatively you could run this validation elsewhere where you can programatically check each row one at a time.
Refer to the user guide about working with tables:
The sub section explaining the SetTable Row function would be relevant here and will give you a lot of control on how you can manage tables
Having said that, where you say this table has 25rows.. I wonder if you may be better off not using table but use Check Box Groups and text input fields where you will then be able to use the default ‘mandatory answer’ on the check box object