Answer:
Add "huddled behind a tree" after "friends."
Step-by-step explanation:
Sentence fragments are when some parts of a sentence are provided or given. They do not form a complete sentence and must be added with other words or phrases to be complete.
In the given sentence fragment, if we add "huddled behind a tree" to it, it will become a full sentence. This means that the sentence fragment given is just one part of the sentence and must be added with the words "huddled behind a tree" to be made into a full, meaningful sentence. It becomes "Bart and Marvin and their friends huddled behind a tree".
Adding "terrified" before Bart will still make the sentence incomplete.
Adding a comma after "friends" also makes no sense.
Likewise, adding "from school" still does not make the sentence complete.
Thus, the correct answer is the third option.