Answer:
I would say that all of the options would be helpful in creating a more formal and professional tone. The correct answer then would be the last one: all of the above.
Step-by-step explanation:
All of the options would make the letter better, and by better I mean more formal. The one that needs to be done is removing the slang, but all the other would also help make it more formal. When requesting help for a formal cause, we should take care of all the details in order to make it as credible as possible, because the cause needs to be taken care of with responsibility. Also, you need to be clear, that is why the sooner you explain your idea and what is your request, the better.