NMOS Transistor Voltage Range For Specification?
You don't know VDS or ID. But your task is to obtain a resistance of 200 to 1000 Ohms. 

That means, depending on how your "variable resistor" will be used, in your final solution you need to have either

\$200 &lt; \dfrac{V_{DS}}{I_D} &lt; 1000\$

or 

\$ 200 &lt; \dfrac{\mathrm{d}V_{DS}}{\mathrm{d}I_D} &lt; 1000\$