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A
Groups are always fine
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B
An adult may buy alcohol and pass it to a minor in the group, which is illegal
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C
Minors cannot enter establishments
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D
Groups get a discount
Why this is the answer
A server should be cautious with mixed-age groups because an adult of legal age might purchase alcohol and then pass it to an underage member of the group — a practice that is illegal and that the server can be held responsible for if they reasonably should have known. Watching for this kind of 'second-party' or 'straw' purchase, and refusing service when it appears alcohol is being obtained for a minor, is part of responsible service. The seller's duty does not end once an of-age buyer is identified.
Source: Responsible Beverage Service — Second-Party Sales