What does an ACK indicate?

• A. The server is starting a handshake.
• B. The connection is established.
• C. The connection is refused.
• D. The port is closed.

Answer: (B)

An ACK in TCP is a confirmation that data has been received and the sender can move forward with the next step. In the connection setup, the final ACK completes the three-way handshake (SYN -> SYN-ACK -> ACK), and at that moment the connection is established, so both sides can start exchanging data. After the connection is established, ACKs continue to confirm receipt of subsequent data segments.

The other options don’t fit because starting a handshake is initiated with SYN signals, not an ACK; a refusal is typically signaled by a reset (RST) rather than a plain ACK; and an ACK on its own doesn’t indicate that a port is closed—the port status is determined by other control messages (like RST or the absence of a handshake).