Explanation

(c) Let the total volume of ice-berg is $V$ and its density is $\rho$. If this ice-berg floats in water with volume $V_{in}$ inside it then $V_{in}\sigma g = V\rho g \Rightarrow V_{in} = \left( \frac{\rho}{\sigma} \right)V\ \lbrack\sigma =$ density of water]
$\Rightarrow V_{out} = V - V_{in} = \left( \frac{\sigma - \rho}{\sigma} \right)V$
$\Rightarrow \frac{V_{out}}{V} = \left( \frac{\sigma - \rho}{\sigma} \right) = \frac{1000 - 900}{1000} = \frac{1}{10}$
$\therefore V_{out} = 10\%$ of $V$