Final answer:
The sentence suggesting the organization of a clean-up day best represents the letter's purpose as it proposes a solution to the littering issue while acknowledging student responsibility.
Step-by-step explanation:
The sentence that best identifies the letter's purpose from the provided paragraph is "Since we helped make the mess, we should organize a clean-up day." This sentence directly addresses action, indicating a resolution and a call to action, which fulfills the purpose of the letter by proposing a solution to the problem. Furthermore, it encapsulates the responsibility felt by the students and their willingness to correct their actions, which aligns with the overarching theme of environmental stewardship.