Answer:
The answer is C.
Step-by-step explanation:
Email is an electronic method of sending mail to a friend, family, colleague, business partners, suppliers etc.
According to this question, the essential element of a formal email is Subject line(option C). The subject line give us the clue of what will be in the body of the mail. The subject line is like the title or the headline.
Option A is not correct because it is not usually in the mail. And this is the reason for the other incorrect options too.