Final answer:
The correct answer is 'vacation'. This implies that the speaker and their classmates made plans for a trip or holiday before the summer ends.
Step-by-step explanation:
The correct answer to complete the sentence 'My classmates and I planned our next _______ before summer ends.' is vacation. This implies that the speaker and their classmates made plans for a trip or holiday before the summer ends. It could be a family vacation, a trip with friends, or any other kind of getaway. It's important to convey thoughts respectfully and maintain the rapport between the individuals in the conversation. Based on the context of the dialogue, which revolves around the sensitive subject of owed money, one can infer the diplomatic phrases needed to fill in the gaps.