Acrobat handles multiple selection in a slightly different way. What you want to do in Acrobat is to add a Radio button and make it part of a button group. To do this you select the Radio Button tool, click on the location in the pdf that you want to place the radio button, and in the Radio Button Choice dialog that appears be sure to give the Radion Button a group name and be sure to check the Required Field checkbox.
Thank you for helping me.
Unfortunately I'm not sure this is what I'm looking for.
If I use the Radio Button, every selection are mutually exclusive.
What I need is to be able to select several responses, but to have to select at least one of them.
People may have the choice of responding by checking one, two or more items but they must answer the question.