Which TCP option enables selective retransmission of segments?

• A. MSS
• B. Window Scale
• C. SACK
• D. Timestamps

Answer: (C)

Selective retransmission is enabled by the SACK option. SACK lets the receiver report exactly which blocks of data have arrived, including noncontiguous segments, in addition to the final cumulative acknowledgment. With this detailed feedback, the sender can retransmit only the missing pieces instead of re-sending everything in the window, making loss recovery faster and more efficient.

The other options serve different purposes: MSS defines the largest segment that can be sent; Window Scale extends the receive window to allow bigger windows on high-bandwidth, long-delay paths; Timestamps provide timing information to estimate RTT and help protect against old duplicate segments. None of these convey precise information about which data blocks have been received, so they don’t support selective retransmission.