Question

read the source of the allusion.
in greek mythology, achilless mother dips him in a river that protects his body wherever it touches. his heel does not get wet, so it is the one part of his body left unprotected. during the trojan war, an arrow hits achilles in the heel and kills him.
what is the meaning of the allusion in the sentence below?
red velvet cupcakes were estelles achilless heel when she was trying to eat more healthily.
a source of strength
a sole weakness

Explanation:

In Greek mythology, Achilles' heel was his only vulnerable part. In the sentence, red - velvet cupcakes are Estelle's only weakness when trying to eat healthily, similar to Achilles' heel being his sole vulnerability.

Answer:

a sole weakness