What field in a TCP segment is used to determine if an arriving data unit exactly matches the data unit sent by the source?

a. Checksumb. DiffServc. Acknowledgment numberd. Source port

a. Checksum

Explanation:

Based on the information provided within the question it can be said that the field that represents this information is called the Checksum. Like mentioned in the question this refers to a value that represents the amount of bits in a certain transmission message. This is done in order to make sure that the amount of bits that were sent match the amount that were received in order to make sure that no data was lost in transit that would cause high-level errors.