Consider carrier transport in a Zener diode in the breakdown region. Which is the dominant transport mechanism for current flow in this case?

The correct option is Tunneling.

A Zener diode is a heavily doped semiconductor p-n junction diode designed to operate in the reverse breakdown region. Due to the high doping concentrations on both the p-side and n-side, the depletion region width is extremely narrow, typically on the order of a few nanometers.

When a reverse bias voltage is applied and approaches the Zener breakdown voltage, an extremely high electric field is established across this very narrow depletion region, often exceeding $10^6\text{ V/cm}$.

This intense electric field causes a significant bending of the energy bands, bringing the conduction band on the n-side very close in energy and space to the valence band on the p-side. Under these conditions, electrons in the valence band of the p-side can quantum mechanically tunnel through the narrow potential barrier of the depletion region directly into the empty states in the conduction band of the n-side. This quantum mechanical tunneling of carriers is the dominant transport mechanism responsible for the sudden and large current flow in a Zener diode during breakdown.