From LimeSurvey Manual
Minimum answers (min_answers)
This function can be used to ensure that a user selects at least a certain number of answers in a multiple choice question.
- Any integer value above 0
- Any number
- Any SGQA code
- Any question code (variable name)
- Any equation, using ExpressionScript syntax, without surrounding curly braces.
If set to X (a random number), the users can submit the survey form if they select at least X checkboxes. If this option is utilised for ranking questions, this ensures that at least three items are ranked before allowing the respondent to submit the answers/survey.