Answer:
C. Subject box
Step-by-step explanation:
The CC box is the carbon copy box. This is where you put other people's emails. They will receive a copy of the email, and they will be able to see who else received the copy.
The BCC box is the blind carbon copy box. This is also where you can put other people's emails. They will receive a copy, but they can't see the others who also got the copy, hence the name blind carbon copy.
The subject box is where you write the subject or topic of the email. This tells the recipients what the email is about, before they begin reading it.
Therefore, the best choice is C: subject box.