Q.Why does zener diode work in reverse bias?

Excess current -> excess heat -> the tiny junction overheats and its structure is destroyed (thermal runaway).

A semiconductor device (such as a diode or transistor) has a very small junction area and mass compared to typical circuit components, so it has a limited capacity to dissipate heat. When a strong current flows through it, the power dissipated as heat ($P = I^2R$) can raise the junction temperature rapidly. In a semiconductor, higher temperature also generates more thermally-excited charge carriers, which further increases the current — a positive-feedback effect called thermal runaway — until the temperature rise permanently damages or destroys the crystal structure/junctio …

A zener diode's useful, voltage-regulating breakdown only happens under reverse bias.

A Zener diode is a heavily-doped p-n junction diode designed so that, under reverse bias, once the reverse voltage reaches a certain sharply-defined value (the Zener voltage $V_Z$), the reverse current increases very sharply while the voltage across the diode stays essentially constant at $V_Z$. This constant-voltage behaviour under reverse breakdown is exactly what makes it useful as a voltage regulator: any device connected across it sees a stable voltage regardless of variations in the supply or load current. In forward bias, the diode behaves just like an ordinary p-n junction diode (conducting once the barrier potential is exceeded) and shows n …

A Zener diode is designed to operate in reverse breakdown at a precise, low voltage. This requires both sides to be heavily doped so the depletion region is very thin, enabling a sharp, controlled breakdown. The correct option is (C).

Avalanche breakdown is a chain-reaction process triggered by high-energy carrier collisions with lattice atoms, distinguishing it from Zener breakdown's direct field-ionisation mechanism.

In a p-n junction diode under large reverse bias, avalanche breakdown occurs in lightly-doped junctions with a relatively wide depletion region. The small reverse saturation current present in the junction consists of minority charge carriers (thermally generated electron-hole pairs). Under the strong reverse electric field, these carriers are accelerated to high velocities and hence high kinetic energies as they cross the depletion region.

When these energetic carriers collide with the semiconductor's lattice atoms, they transfer enough energy to break covalent bonds and knock valence electrons free, creating new electron-hole pairs (impact ionisation). These newly-freed carriers are, in turn, also accelerated by the field and cause further collisions, releasing still more carriers -- a multiplying chain reaction known as avalanche multiplication. The result is a sudden, very large increase in reverse current: avalanche breakdown.

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