Final answer:
TCP provides flow control using the window size field in its header, which specifies the allowed amount of unacknowledged data in transit to manage the data flow and ensure reliable communication.
Step-by-step explanation:
The field in the transport layer header that TCP can adjust to provide flow control is the window size. Flow control is a mechanism that ensures a sender does not overwhelm a receiver by sending data faster than it can be processed and acknowledged. The window size specifies the amount of data that a sender can transmit without receiving an acknowledgment from the receiver. If the receiver's buffer is getting full, it can advertise a smaller window size to the sender, effectively asking the sender to slow down. This helps in managing the flow of data and ensures reliable communication between sender and receiver.