It is either C or, as you said, D.
The standard American telephone number is ten digits, such as (555) 555-1234. The first three digits are the "area code," which, in the past, indicated in what part of the country the phone was located.
The regular American telephone number has 10 digits, such as (999) 000-0000. The nines, or 3 digits, are the area code of the phone.
They are required numbers that are based off of where the phone is, the area code.
