Answer:
The correct answer for the given question is option(b) i.e " No subject line"
Explanation:
In the e-mail subject is the important part which describe the information which kind of particular mail is send or received .if we do not give any subject line in the e-mail header then most of the chances that email will not read by recipients due to this suspicion will be caused.
Unknown Sender and Multiple recipients do not cause suspicion so the correct answer is option(b) i.e No subject line".