Question

a file that is sent along with an email message is called a(n):
selected file
attachment
document
add - on

Explanation:

In email terminology, a file sent along with an email message is known as an attachment. A "Selected file" is just a file that has been chosen, not specifically related to email sending. A "Document" is a type of file but not the term for a file sent with an email. An "Add - on" is a software component that adds functionality, not a file sent with an email.

Answer:

B. Attachment