Which sentence from the paragraph best identifies the letter’s purpose? the woods behind the high school have become littered with trash. the students at the school are partly responsible for the problem. we are often careless with our trash when we are in the schoolyard. since we make the mess, we should organize a clean-up day.

The correct answer is the final sentence: 'Since we helped make the mess, we should organize a clean-up day'. In order to determine the purpose of a piece of writing, ask yourself why the author wrote it.

Why did the author write this paragraph? 
The author wrote this paragraph to persuade the high school to organize a clean-up day.

Now that we know the purpose of the paragraph, we can decide which of the sentences best reflects that purpose. Sentences 1, 2, and 3 provide information and describe the woods behind the school, but don't really talk about the author's purpose. The final sentence, however, is the call to action. This sentence is meant to persuade the high school to organize a clean-up day, therefore it identifies the author's purpose.

Hope this helps!