From LimeSurvey Manual
This option allows the survey administrators to request their respondents to answer to certain survey questions. If the mandatory questions are not answered, the respondents will not be able to proceed further. If you have a question with multiple subquestions and you require only certain subquestions to be answered, use the minimum answer attribute located under the Logic tab.
If you use any of the preinstalled themes and the mandatory question attribute is enabled, a red asterisk will be shown next to the question. If you wish to hide it, please check these instructions.
- On - Question must be answered before the participant can proceed to the next page - the answer option 'No answer' is never shown.
- Soft - If the question is not answered a warning is shown when trying to proceed to the next page - however the participant can choose to ignore the warning and proceeed. Note that the 'No answer' option is still shown (if activated in survey presentation settings)
- Off (default) - Question can be left unanswered