(d) Here, $l = 1m,\ \theta = 30{^\circ},\ g = 9.81\ \ ms^{- 2},\ t = ?$
$t = \sqrt{\frac{2l(1 + K^{2}/R^{2})}{g\sin\theta}}$
For a rupee coin, $K^{2} = \frac{1}{2}R^{2}$
$t = \sqrt{\frac{2 \times 1(1 + 1/2)}{9.81\sin{30{^\circ}}}} = \sqrt{\frac{6}{9.81}} = 0.78\ s$