a
Equivalent resistance $R = 4 + \frac{{3 \times 6}}{{3 + 6}} = 6\,\Omega $ and main current $i = \frac{E}{R} = \frac{3}{6} = 0.5\,A$
Now potential difference across the combination of $3$ $\Omega$ and $6$ $\Omega$, $V = 0.5 \times \left( {\frac{{3 \times 6}}{{3 + 6}}} \right) = 1\,Volt$
The same potential difference, also develops across $3$ $\Omega$ resistance.