The correct answer is D. all of the above.
The relay operating speed depends upon the spring tension, the rate of flux built up, and the armature core air gap.
The spring tension determines the amount of force that is required to move the armature. The higher the spring tension, the more force is required to move the armature, and the slower the relay will operate.
The rate of flux built up determines how quickly the magnetic field is created when the relay is energized. The faster the flux builds up, the faster the armature will be pulled into the magnetic field, and the faster the relay will operate.
The armature core air gap determines how much resistance there is to the movement of the armature. The larger the air gap, the more resistance there is, and the slower the relay will operate.
In conclusion, the relay operating speed depends upon the spring tension, the rate of flux built up, and the armature core air gap.