The correct answer is D. All of the above.
The angular displacement between two interconnected stations is mainly due to the armature reactance of both alternators, the reactance of the interconnector, and the synchronous reactance of both the alternators.
The armature reactance of an alternator is the reactance of the windings that carry the current that produces the magnetic field. The reactance of the interconnector is the reactance of the conductor that connects the two alternators. The synchronous reactance of an alternator is the reactance of the magnetic field that is produced by the current in the armature windings.
The angular displacement between two interconnected stations is the angle between the two voltage phasors at the two stations. The voltage phasor at a station is the vector sum of the voltage phasor due to the armature reactance, the voltage phasor due to the reactance of the interconnector, and the voltage phasor due to the synchronous reactance.
The angular displacement between two interconnected stations is important because it determines the power flow between the two stations. The power flow between two stations is proportional to the cosine of the angular displacement between the two stations.