Which statement about the TCP window size is true?

• A. The window size controls the maximum number of open connections.
• B. The window size defines how many bytes a sender can transmit before receiving an acknowledgment.
• C. The window size defines the total size of the TCP header.
• D. The window size defines how many bytes can be sent before an acknowledgment; larger windows can increase throughput but are constrained by network conditions and congestion.

Answer: (D)