Answer:
C. The email will not be transmitted without that information.
A subject line is important in an email because it gives the receiver an idea of what the email is about and whether or not it is important or relevant to them. Leaving the subject line blank can cause confusion, delay the receiver opening the email or even mark it as spam, and make the receiver not consider the message important. But an email will be transmitted without a subject line, it's just that the receiver may not understand the content of the email.