Correct Answer - Option 3 : 28.56 kΩ
Voltmeter reading = 250 V
Ammeter reading = 10 mA
Hence, effective value of resistance
\({R_{eff}} = \frac{{250\;V}}{{10\;mA}} = 25\;k{\rm{\Omega }}\)
Let us say that the value unknown resistance is R (the unknown resistance and the internal resistance will be in parallel) then,
Reff = R ∥ 200 kΩ
⇒ \(25\;k{\rm{\Omega }} = \frac{{R \;\times\; 200}}{{R\; + \;200}}\)
⇒ R = 28.56 kΩ