It is a misdemeanor offense, so by legal definition it is "minor". However, I would disclose it if the nature of the question is trying to reveal what criminal record you have. If you are found later to have not disclosed what they think you should have, it could have worse consequences than if you would be open about it at the beginning. You may want to talk with someone in the administration on a more personal level about your matter.
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You might try first reading the application instructions closely. If the college is in another state, the legal distinction between major and minor and Montana's misdemeanor and felony categories might not be the same for a DUI per se. If the instructions do not make this clear, ask as advised above.
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