Which of the following guidelines about the subject line of e-mail messages is most appropriate?

make sure you provide all the details in the subject line in case the receiver does not have time to read the whole e-mail.
use the subject line wisely to let your reader know the main purpose of the e-mail.
try to keep the subject line very short, such as just one or two words.
avoid using a subject line unless the e-mail is very important.

The second option, "Use the subject line wisely to let your reader know the main purpose of the e-mail," is most appropriate.

You can find this answer by eliminating the options -- the first option doesn't make sense because if you include all the details in the subject line, it will be too long. The third option, one or two words, would not help the reader know what's in the email. And the last option, putting nothing in the subject line, would also not help the reader know what's in the email.