Avalanche breakdown is primarily dependent on the phenomenon of:
A. Collision
B. Ionisation
C. Doping
D. Recombination

Hint: For this question, we will discuss what the process of avalanche breakdown is. How it happens. In what kind of materials it can happen. What are the after effects once avalanche breakdown takes place in a material.

Complete step by step answer:
During avalanche breakdown, charge carries inside a semiconductor or an insulator are accelerated under a very high electric potential. Due to this the charge carriers acquire a very high velocity which leads them to colliding with more atoms and release of more free electrons which makes more charge carriers available and they in turn collide with several more atoms to make more charge carriers. In this way a chain reaction is formed, it is similar to an avalanche, hence the name. Once avalanche breakdown happens in a material, the charge carriers remain there, and the material will still conduct even if the potential difference applied is reduced. This is a similar phenomenon as electron avalanche by which gases can be turned into electrically conductive plasma. This is used in semiconductors for making high voltage Zener diodes. Due to this effect after breakdown of high voltage Zener diodes, current passes uniformly through the diode and they can indefinitely sustain such levels of current during breakdown and no permanent damage is caused.

As this whole process occurs due the collisions, the correct option will be A.

Note: Students should not confuse between avalanche breakdown and Zener breakdown or reverse breakdown of a diode. Avalanche breakdown can also happen in insulators unlike the other two. Also, Zener diodes stop conducting if voltage drops below their breakdown voltage. But the current flow continues in avalanche breakdown even if the voltage drops.