The $6 \mathrm{~V}$ Zener diode shown in given figure has zero Zener resistance and a knee current of $5 \mathrm{~mA}$. The minimum value of $R$ so that the voltage across it does not fall below $6 \mathrm{~V}$ is
- $1.2 $ ohms
- $80$ ohms
- $50$ ohms
- $0$ ohms