Refer to figure.
EASA learning objective 022.04.04.01.01 states: 'Explain the purpose of the directional gyroscope.'
EASA learning objective 022.04.04.01.03 states: 'Explain how the directional gyroscope will drift over time due to the following: rotation of the Earth; aircraft manoeuvring; aircraft movement over the Earth’s surface/direction of travel.'
EASA learning objective 022.04.04.01.04 states: 'Describe the procedure for the pilot to align the directional gyroscope to the correct compass heading.'

All aircraft are fitted with a direct reading magnetic compass (DRC). Unfortunately the DRC is prone to indication errors during acceleration and turns and is not very intuitive to use. A directional gyroscope (DG) overcomes these problems by using the rigidity of a horizontally tied gyro to give a steady gyro heading datum and is the main source of heading information in light aircraft.

As there is no external heading input to the DG the pilot must align it with the compass heading indication on the aircrafts DRC before getting airborne. The gyro in the DG will drift over time due to real wander caused by aircraft manoeuvres and apparent wander caused by Earth rotation and transport drift. Because the DG is only accurate for a short period of time it must be manually resynchronised with the DRC heading at regular intervals (every 10 mins).

This synchronisation is achieved by pushing in the caging knob and turning it to reorient the display to the compass heading on the DRC. Synchronisation must be done in straight, level, unaccelerated flight to ensure that the indications of the DRC are correct and the gyro axis is horizontal.

(1) true; (2) automatically; (3) long --> INCORRECT. The DG has to be manually synchronised with the DRC which indicates compass heading not true heading. It's only accurate for a short period due to drift.

(1) magnetic; (2) manually; (3) short --> INCORRECT. The DG is synchronised with the DRC which indicates compass heading. Magnetic heading has to be derived by applying deviation which varies with compass heading.

(1) magnetic; (2) automatically; (3) long --> INCORRECT. Due to the reasons stated above.

(1) compass; (2) manually; (3) short --> CORRECT. The DG is manually synchronised with the DRC which indicates compass heading and is only accurate for short periods due to drift.