In the given circuit if the power rating of Zener diode is \( 10\,\mathrm{mW} \), the value of series resistance \( R_s \) to regulate the input unregulated supply is:
\( 10\,\text{k}\Omega \)
\( 1\,\text{k}\Omega \)
\( 10\,\Omega \)
\( 5\,\text{k}\Omega \)