Take another look at the passage:
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 third part, An explosive belch erupted
from his mouth. This part has a main
clause: Belch is the
is the verb. A
fragment will not have a main clause.
Go back to the passage to try again.