Take another look at the item:

With the huge number of confusing rules colliding in her head, (A) Cindy took the quiz on fragments (B) and hoped, (C) that she would remember enough of the material to pass.

You wanted to do this:

With the huge number of confusing rules colliding in her head. Cindy took the quiz on fragments and hoped, that she would remember enough of the material to pass.

The comma after head is correct. Substituting a period for that comma would make the opening phrase a fragment.

You might want to review the rules.

Go back to the sentence to try again.

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