Which of the following types of scan does not open a full TCP connection?

A FIN scan sends a FIN flag to a closed port to elicit a RST response for port discovery and also does not open a full connection, but its defining characteristic is probing closed ports via out-of-state packets, not the half-open handshake property.

An ACK scan sends ACK packets to map firewall stateful rules and also avoids a full connection, but its purpose and defining trait is firewall rule enumeration rather than half-open connection avoidance.

A stealth scan, also called a half-open or SYN scan, sends a SYN packet and upon receiving a SYN-ACK immediately responds with a RST to abort the session before a full TCP connection is established. Because no complete connection is logged by the target application layer, this is the canonical technique associated with avoiding a full TCP connection in CEH and port-scanning terminology.

An idle scan routes spoofed SYN packets through a zombie host and also avoids a direct full TCP connection, but its defining property is using a third-party host to anonymize the scan origin.