Correct answer is the second option: "The letter was <em>sent</em> by a disgruntled customer."
The first option refers to the decimal unit.
The third option refers to arranging for the transport of something.
The last option refers to a smell.
Hello! I have one suggestion that would help your essay stand out and make a greater impact. Your claim is very spot on and shows what you want to address. There are some restructuring points that can take place to better the grammar of the essay and make it sound more pulled together, especially where you state "Repetition, the quote..." can be restated as "Orwell utilizes repetition throughout the novella to further impact the overall persuasiveness of the language that he uses to convey his stances. He uses the quote "long live the windmill! Long live Animal Farm!" in order to emphasize a call-to-action and encourage for the speeches that the leaders use to give the other animals an incentive to protect Animal Farm."
If you use this same structure, it ties everything back to your claim and fully emphasizes what you want to state clearly. Just fix the spots where you state the rhetorical device and add more detail in order to fully tie everything back!
Good luck! If this is for AP Language, you're in for a great experience!