Answer:
at the beginning of the third sentence
Explanation:
We can say that a letter that begins with <em>I </em><em>need </em><em>your </em><em>help </em><em>ASAP </em>is informal because it is personal and direct.
We know that beacuse the sentence has been written to a trusted recipient and it uses abbreviations (ASAP).
D) Always have a topic sentence first