Answer:
d. clear subject line, concise messaging, and appropriate salutations
Step-by-step explanation:
When you write an email you need to write professionally. You need to clearly state what your email is about to avoid conclusion. Avoid A because it is extremely unprofessional and can be confusing for people who do not know abbreviations. B is obnoxious because you are mass emailing people and your point in the email is unclear. C would be lengthy and take unnecessary time out of the recipients day. D is perfect because it is professional and it is short and to the point, which is what employers and professors would expect.