Take a look at the passage again:
Joshua found the pressure from the gas in his stomach
unbearable. Although he didn't want to be rude in the middle of geology
class. An explosive belch erupted from his mouth.
You chose the first part, Joshua found the pressure from the gas in his stomach unbearable. This part has a main clause: Joshua is the subject; found is the verb. A fragment will not have a main clause.
Go back to the passage to try again.