Which of the following is considered as one of the most reliable forms of TCP scanning?

TCP Connect/Full Open Scan uses the OS to complete the full SYN, SYN-ACK, ACK handshake before sending a RST to close the connection. Because the connection is fully established, the result is definitive - there is no ambiguity about whether a port is open. This reliability comes at the cost of being easily logged by the target system.

Half-open (SYN) scan never completes the handshake, so it is stealthier but can produce ambiguous results if SYN-ACK packets are dropped or filtered.

NULL Scan sends packets with no TCP flags set and relies on RFC-compliant RST responses, but non-RFC-compliant systems such as Windows do not respond as expected, making it unreliable.

Xmas Scan sets the FIN, URG, and PSH flags simultaneously and suffers the same reliability limitation as NULL Scan - it fails against Windows hosts and non-RFC-compliant stacks.