Looking at the answers:

'When the entire aircraft is beyond the runway-holding position.' --> CORRECT. The call of 'runway vacated' can only be made when the ENTIRE aircraft has crossed the holding line. From an ATC point of view, this now frees the runway to be used by another aircraft.

'When the aircraft starts turning to take the exit taxiway.' --> INCORRECT. The aircraft is turning off the runway centreline but still occupies the runway itself.

'As soon as the aircraft is on the exit taxiway and at taxi speed.' --> INCORRECT. If you are on the exit taxiway and have entirely crossed the holding point, then the runway is vacated. The aircraft could be taxiing or stationary while awaiting further clearance. It does not need to necessarily be at taxi speed.

'As soon as the aircraft has crossed the runway-edge line.' --> INCORRECT. The aircraft is still occupying the runway in this position and blocking it's use by another aircraft. The holding points ensure proper separation for another aircraft to use the runway.

ICAO Doc 4444
7.10.3 Landing and roll-out manoeuvres

7.10.3.4 When necessary or desirable, e.g. due to low visibility conditions, a landing or a taxiing aircraft may be instructed to report when a runway has been vacated. The report shall be made when the entire aircraft is beyond the relevant runway-holding position.