Which of the following semi-conducting devices is used as a voltage regulator?

When the Zener diode is used as a voltage regulator,it is connected in

In the given circuit diagram,a $5 \,V$ Zener diode along with a series resistance $R$ is connected across a $50 \,V$ power supply. If the maximum Zener current is $90 \,mA$,the minimum value of the resistance $R$ required will be $..... \,\Omega$.

$A$ Zener diode, having a breakdown voltage of $15 \text{ V}$, is used in a voltage regulator circuit as shown. The current through the Zener diode is (in $\text{ mA}$)

$A$ Zener diode with a breakdown voltage of $15 \ V$ is used as a voltage regulator. In the circuit shown,the current flowing through the Zener diode is ...... $mA$.

$A$ Zener diode of breakdown voltage $20 \ V$ is connected as shown in the given circuit. The current through the Zener diode is (in $mA$)