Review the passage:
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 could not outpace Mother Nature.
You chose the first part, While thunder boomed in the distance, Margaret started off on her daily five-mile run. This part has a main clause: Margaret is the subject; started is the verb. A fragment will not have a main clause.