Answer:
D
Explanation:
A) Too casual of a greeting, this is a professional
B) Too casual of a greeting, this would be more how you would text or email a friend/peer
C) Does not adress the author at all
D) Respectful greeting. After this, you would ask your question.
Example: I was curious as to where you got your ideas for the book from. I'm writing a report and would love to know how you came about...(and so on)
You would also want to make sure to sign off the email respectfully using things such as Best regards or SIncerely and put your name.