Based on historical context, the holiday treat that was a vehicle for alcohol during prohibition is "Eggnog."
History of Eggnog
Originally, Eggnog is a form drink that contains eggs, milk, and sugar, majorly as an ingredient. The history of Eggnog can be traced to England.
However, during the prohibition era, which lasted between 1920 to 1933, the Eggnog producers or makers tended to add rum or alcohol to the mix, which gave people a chance to take alcoholic drinks. It is served chiefly during Christmas.
Hence, in this case, it is concluded that the correct answer is "Eggnog."
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