Explanation

(a) Equivalent circuit of the given circuit is
Between points C and D resistors 2$\Omega$, 2$\Omega$ and 2$\Omega$ are in series, therefore, their equivalent resistance,
$R^{'} = 2 + 2 + 2 = 6\Omega$
Resistors R’ and 6$\Omega$ are in parallel, therefore their equivalent resistance is given by
$\frac{1}{R^{''}} = \frac{1}{6} + \frac{1}{6}$
$R^{''} = 3\Omega$
Now between points A and B 1$\Omega$, 3$\ \Omega$ and 1$\Omega$ are in series.
Therefore, resultant resistance is
R=1+3+1=5$\Omega$