Final answer:
The allusion in the sentence 'Red velvet cupcakes were Mary's Achilles's heel when she was trying to eat more healthily' means that the cupcakes were her sole weakness or vulnerability.
Step-by-step explanation:
The allusion in the sentence is 'Achilles's heel' which refers to a sole weakness or vulnerability. In Greek mythology, Achilles was a warrior who was invulnerable, except for his heel. The phrase 'Achilles's heel' has come to represent a person's weak point or something that can cause their downfall. In the given sentence, it means that red velvet cupcakes were Mary's only weakness when she was trying to eat more healthily. No matter how hard she tried, she couldn't resist them.
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