Answer: Code bits.
Step-by-step explanation:
HDLC (High-level Data Link Control) is a protocol used for transmitting data over a communication link. The HDLC frame consists of several fields, including the control field, which contains information about the flow and control of data. The control field in HDLC always includes three code bits, which identify the type of frame and the status of the communication. These code bits are always present in the control field, regardless of the type of frame or the data being transmitted. The other fields in the control field, such as N(S) and N(R), are used for sequence numbering and acknowledgment of frames, respectively, but they may not be present in all frames. Therefore, the correct answer is "Code bits."