Take a look at the passage again:
To stay awake for a long evening of chemistry homework. Beth made a huge
pot of coffee. Unfortunately, her heart beat so
fast and her legs jiggled so violently that the poor girl couldn't concentrate.
You chose the third part, Unfortunately, her heart beat so fast .... This part has a main clause: heart is the subject; beat is the verb. A fragment will not have a main clause.
Go back to the passage to try again.