Here is the item again:
Despite Cassandra's stern warning, Alex walked over to flirt with Helen.
The most beautiful—and dangerous—woman at the party.
You chose to use Punctuation Rule 9 to fix the problem:
Main Clause + , + Appositive.
Here is the correction:
Despite Cassandra's stern warning, Alex walked over to flirt with
Helen,
the most beautiful—and dangerous—woman at the party.
Well done! You deserve that cyber chocolate bar!