Review the original item:
Bo was craving a cold slice of watermelon, but he had money only for hot
dogs and buns,
Bo was pretty sure that sticking the huge fruit under his shirt and
pretending that he was pregnant would not fool the cashiers.
You chose the correct way to fix the error:
Bo was craving a cold slice of watermelon, but he had money only for hot
dogs and buns. Bo was pretty sure that sticking
the huge fruit under his shirt and pretending that he was pregnant would
not fool the cashiers.
He had money only for hot dogs and buns is a main clause. Bo was pretty sure is the second main clause.
You are always correct when you use a period and a capital letter between the two main clauses that create the comma splice.