Hi @Julian_gp and welcome to the forum! When you get a chance, please introduce yourself on this forum thread.
Can you provide some more details? Do you mean a question with a list of answer options, and the respondent can select more than one option for their answer?
If that is the case, you will want to add a "Select Multiple" type question.
You might want to try using XLSForm. It takes a little bit more work to learn than ODK Build, but it makes it easier to create more complex forms. Read more about it at these two links: