Which measure would be both practical and increase airline safety the most?

Two always in the cockpit, either two pilots or pilot and a guard as required.

Have three pilots on a flight, and two of them must always be in the cockpit.

The cockpit door is able to also be unlocked from the outside.

Increase the psychological screenings and mental health support for pilots.

Two people always in the cockpit, and both must be pilots.

Controls which require two operators to simultaneously alter the flightpath.