$A$ Zener diode acts as a/an

In the following circuit,the reading of the ammeter will be (Take Zener breakdown voltage $= 4 \ V$) (in $mA$)

$A$ voltage regulating circuit consisting of a Zener diode, having a breakdown voltage of $10 \ V$ and a maximum power dissipation of $0.4 \ W$, is operated at $15 \ V$. The approximate value of the protective resistance in this circuit is . . . . . . $\Omega$.

For the given circuit,the power across the zener diode is ..... $mW$.

The graph given below represents $I-V$ characteristics of a Zener diode. The part of the characteristics curve that is most relevant for its operation as a voltage regulator is

If the current in the Zener diode is five times the current in $R_1$ and the breakdown voltage of the diode is $6 \text{ V}$,find the value of $R$.