Under the answer option in question properties you can check the multi answer instead of single answer, this will change radio to check boxes. for one selection you can apply custom validation.

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If you want to keep it as a multi-select question then simply select "Multiple Answer" from "Answer property". If you want to show check-box but still want the user to select only one then there are 3 ways



1) Make all check-box exclusive: https://www.qualtrics.com/support/survey-platform/survey-module/editing-questions/formatting-answer-choices/#MakeAnswerExclusive

2) Add custom validation: https://www.qualtrics.com/support/survey-platform/survey-module/editing-questions/validation/#CustomValidation

3) Through JavaScript

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