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

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

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

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

Increase the psychological screenings and mental health support for pilots.

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

Controls which require two operators to simultaneously alter the flightpath.