In which cases must you activate the release valve?

You answered correctly. The release valve must be activated when the air tanks are full, the brake lines are not connected, and you need to move the trailer. In this situation, pressure in the air tanks can be released; otherwise, the trailer’s brakes would remain engaged because the parking brake (which uses air pressure to release) cannot be operated without connection to the towing vehicle. The other answers are not correct because you do not need to activate the release valve to fill the tanks or to release the brakes before starting your trip—those steps require connecting the brake lines so proper air pressure can be supplied to the trailer's system. Remember, the release valve is specifically a backup solution when the trailer needs to be moved without being connected to a tractor.