Which TCP optimization prevents unnecessary retransmissions and improves throughput?

Selective Acknowledgment (SACK) is a TCP optimization that allows a receiver to inform the sender about all segments that have been successfully received, even if they are non-contiguous. This prevents the sender from retransmitting segments that have already arrived but might be out of order, thus reducing unnecessary retransmissions and improving overall throughput, especially in environments with packet loss. SEQ refers to sequence numbers, which are fundamental to TCP but not an optimization in themselves. RTT (Round-Trip Time) is a measurement used for congestion control and retransmission timers, not an optimization method. SYN is part of the TCP three-way handshake and not an optimization for retransmissions.