Explanation

(d) Resistance R bisecting the circuit can be neglected due to the symmetry of the circuit.
Now, there are four triangles
Effective resistance of each triangle
$\frac{1}{R^{'}} = \frac{1}{R} + \frac{1}{2R}$
$\ \ \ \ \ \ \ = \frac{2 + 1}{2R} = \frac{3}{2R}\ $
$\therefore R^{'} = \frac{2}{3}R$
Now the given circuit reduced to
Therefore, effective resistance between A and B,
$\frac{1}{R_{AB}} = \frac{1}{2R^{'}} + \frac{1}{2R^{'}} = \frac{1}{R^{'}}$
$\Longrightarrow R_{AB} = R^{'} = \frac{2R}{3}\Omega$