The correct answer is A. The nmcli con mod IFNAME connection.autoconnect no command is used to modify an existing network connection to not automatically connect at system boot, where IFNAME needs to be replaced with the actual connection name or UUID. Remember to use 'con mod' for modifying an existing connection, which retains the interface configuration for future use. Answer B, using 'nmcli dev disconnect IFNAME', is incorrect, as it only disconnects an active connection but does not change the autoconnect property of the connection. Answer C, 'nmcli con up IFNAME', is the opposite of what is needed as it activates a connection. Answer D, 'nmcli con delete IFNAME', permanently removes the connection profile, which is not suitable when the goal is to keep the profile for later manual activation.