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

TCP Connect scanning completes the entire TCP 3-way handshake (SYN, SYN-ACK, ACK) using the OS's connect() system call, which guarantees a definitive open/closed result for each port. Because the handshake fully completes, it works correctly across all operating systems and network configurations. This reliability comes at the cost of being easily logged by the target system.

Half-open (SYN) scanning never completes the handshake, which can yield ambiguous results in certain firewall or filtering configurations.

NULL scans send packets with no TCP flags set and are unreliable because Windows systems typically do not respond as the RFC specifies, producing false negatives.

Xmas scans set FIN, PSH, and URG flags simultaneously and are similarly unreliable against Windows hosts and non-RFC-compliant stacks.