If the galvanometer reading is zero, in the given circuit. Then the value of ‘X’ will be 

 

No current passes through galvanometer means P.D across 'X' is equal to 2V volt and across 500Ω is equal to 10V.
Current through 500Ω is $\frac{10}{500}=2\times {10}^{-2}\mathrm{A}$ 
${\mathrm{V}}_{\mathrm{x}}={\mathrm{i}}_{\mathrm{x}}{\mathrm{R}}_{\mathrm{x}};2=2\times {10}^{-2}\left(\mathrm{X}\right)\Rightarrow \mathrm{X}=100\mathrm{\Omega }$