A potential difference of 200 V is maintained across a conductor of resistance 100 $\Omega$. The number of electrons passing through it in 1 s is

$\mathrm{I}=\frac{\mathrm{V}}{\mathrm{R}}=\frac{200\mathrm{V}}{100\mathrm{\Omega }}=2\mathrm{A}$
2A of current means 2 coulomb of charge is flowing through conductor at any cross section per second.
Now, 1 electron has charge $1.6\times {10}^{-19}\mathrm{C}$
Charge $1.6\times {10}^{-19}\mathrm{C}$ having 1 electron.
So, 1C charge having $\frac{1}{1.6\times {10}^{-19}}$ electron
Therefore, 2 C charge has number of electron $=\frac{2}{1.6\times {10}^{-19}}=1.25\times {10}^{19}$