Answer:
a) In the subject line
Step-by-step explanation:
Email stands for electronic mail in which the sender sends the email to various receivers who receive the mail.
While composing any type of mail especially day to day emails, the important information or we can say primary message which should be mentioned in the subject line so that the receiver focus on the message send and revert to the sender accordingly
All other options are not appropriate as it does not contains any primary information