(c) $R_{2},\ R_{3}and\ R_{4}$ are in parallel order, so their equivalent resistance
$\frac{1}{R^{'}} = \frac{1}{R^{2}} + \frac{1}{R^{3}} + \frac{1}{R^{4}}$
$\ \ \ \ \ \ = \frac{1}{50} + \frac{1}{50} + \frac{1}{75}$
$\ \ \ \ \ \ = \frac{30 + 30 + 20}{1500}$
$\ \ \ \ \ \ = \frac{80}{1500} = \frac{4}{75}\ $
$\therefore\ \ R^{'} = \frac{75}{4}\Omega$
$R = R_{1} + R^{'} = 100 + \frac{75}{4}$
$\ \ \ \ = \frac{475}{4}\Omega = 118.75\Omega$