Detailed Answer
Answer (C) is correct. A check digit, or self-checking number, is an input control to determine if an error might have been made on an identification number. The digit is an extra number on the end of the identification number (creating a new ID number) that is calculated by an algorithm on the original part of the ID number. If the ID is miskeyed, the algorithm will produce a number different from the check digit and an error will be detected and reported.