Take another look at the passage:
While eating the plate of sesame chicken, Maria was thankful for her skill with chopsticks. Eating utensils that required only one hand to use. For Maria's left hand was pinned to the booth by Hunter, her five-year-old son who lay sleeping after a long day of shopping at the mall.
You chose the third part, For Maria's left hand was pinned to the booth by Hunter .... This part has a main clause: hand is the subject; was pinned is the verb. A fragment will not have a main clause.