What should a driver do to secure the towing vehicle before unhitching a trailer?

Disconnect the electrical supply to the trailer lights and lower the jockey wheel.
Apply the handbrake, switch off the ignition and engage a low gear.
Lower the jockey wheel and then switch off the ignition.
Apply the handbrake, lower the jockey wheel and switch off the engine.

Explantion: Before unhitching a trailer, apply the handbrake, switch off the ignition and engage a lower gear, so that the towing vehicle does not move while you are unhitching the trailer.

Theory Tester

What should a driver do to secure the towing vehicle before unhitching a trailer?

Legal Matters/Rules of the Road