Take a look at the passage again:
While thunder boomed in the distance, Margaret started off on her daily
five-mile run. Confident that she had plenty of time before the storm arrived.
Wet hair and clothes plastered to her body proved
that she couldn't outpace Mother Nature.
You chose the third part, Wet hair and clothes plastered to her body proved that she couldn't outpace Mother Nature. This part has a main clause: hair and clothes are the subject; proved is the verb. A fragment will not have a main clause.
Go back to the passage to try again.