It should have told you that you had "a typo" if nothing else. The only correct verb form here is התפוצצו (hitpotzetzu) because אבטיחים is plural. התפוצצה (hitpotzetzah) would apply to a feminine-gender noun in the singular, so if a car exploded you could say המכונית התפוצצה . (Less fun than the watermelons, though)
I don't think it would be any different in Hebrew. In English, "exploded" suggests it's part of a narrative, while "have exploded" suggests you're talking about a situation that has just happened and whose effects (a lot of sticky pink mess) might well be still present. But Hebrew only has the one past tense, so your answer should be accepted.