Answer: 2: Use the subject line wisely to let your reader know the main purpose of the e-mail.
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Explanation:
The subject line is like the title of a book. You want to give a quick sense of what the email is about, but you also don't want to put the entire contents of the book in the title. The subject line only has so much room. Even less room is available if the reader is checking their email on their mobile device.
Choice 1 is false because subject lines are important. I can't think of a case where you can omit the subject line entirely (ie leave it blank), unless it's a situation where the recipient knows in advance what the topic is about. Though that may be rare.
Choice 3 is false. Yes you want to keep the subject line short, but you also don't want to be cryptic or confusing. If you can get away with using 1 or 2 words, then go ahead and do so. However, you may need more words to convey the topic of the email. Go with what feels right and try to put yourself in the shoes of the person reading your email. Ask yourself "Does this convey the message I want to send?"
Choice 4 is false. As mentioned, you do not want to put the entire email in the subject line. The exception of this rule is that if the email is really really short. For every other case, you should have a fairly short title and then go over the details in the email main body itself.
