Which part of a formal e-mail is optional? the closing the contact information the person’s last name the salutation

The answer is the second choice - the contact information. Of course, if it is a business letter and they need to get in touch with you, that does change the purpose. But as this is a formal letter, not a business letter, it is not an obligation to give an address.

When laying out a formal letter you need to focus on 6 areas:
- Your address, recipient’s address (who you’re sending the letter to), date, salutation, closing, your signature/name.

Under your address are the following:
Street number and street name
Suburb
City
Post Code
Country (only if sending the letter overseas)
Phone number (optional)
Email address (optional)

B- The contact information