Why is there a sudden increase in current in a Zener diode?

Consider the following statements $(A)$ and $(B)$ and identify the correct answer.
$(A)$ $A$ Zener diode is connected in reverse bias when used as a voltage regulator.
$(B)$ The potential barrier of a $p-n$ junction typically lies between $0.1 \, V$ to $0.3 \, V$.

If the voltage across the Zener diode is $6\, V$, then find out the value of the maximum resistance $R$ in this condition. (in $k\Omega$)

$A$ Zener diode is used for

What happens during the regulation action of a Zener diode?

$A$ diode has Zener voltage of $10 \text{ V}$ and maximum power dissipation of $0.5 \text{ W}$. The minimum resistance to be used in series with this diode for safety when it is connected to a $25 \text{ V}$ power supply is . . . . . . $\Omega$.