(c) Here, $\theta = 60{^\circ},\ l = 10\ m,\ a = ?$
$a = \frac{g\sin\theta}{1 + K^{2}/R^{2}}$
For solid sphere, $K^{2} = \frac{2}{5}R^{2}$
$a = \frac{9.8\sin{60{^\circ}}}{1 + \frac{2}{5}} = \frac{5}{7} \times 9.8 \times \frac{\sqrt{3}}{2} = 6.06\ ms^{- 2}$