Explanation

(a) The equivalent resistance between $C$ and $D$ is
$\frac{1}{R'} = \frac{1}{6} + \frac{1}{6} + \frac{1}{3} = \frac{2}{3}$ or $R^{'} = \frac{3}{2} = 1.5\Omega$
Now the equivalent resistance between $A$ and $B$ as $R^{'} = 1.5\Omega$ and $2.5\Omega$ are connected in series, so
$R" = 1.5 + 2.5 = 4\Omega$
Now by ohm’s law, potential difference between $A$ and $B$ is given by $V_{A} - V_{B} = iR = 2 \times 4.0 = 8\ volt$