What is the mandatory method for securing ISO shipping containers onto commercial flatbed skeletal chassis trailers (chasis portacontenedores)?

1. Secured with nylon ropes tied to the side mirrors
2. Locked at all four bottom corner castings using certified mechanical Twist-Locks (trabas twist-lock) fully engaged and locked
3. Held in place solely by the heavy deadweight of the container
4. Taped along the seams with industrial adhesive tape