(c) For the floatation $V_{0}d_{0}g = V_{in}\ d\ g \Rightarrow V_{in} = V_{0}\frac{d_{0}}{d}$
$\therefore V_{out} = V_{0} - V_{in} = V_{0} - V_{0}\frac{d_{0}}{d} = V_{0}\left\lbrack \frac{d - d_{0}}{d} \right\rbrack$
$\Rightarrow \frac{V_{out}}{V_{0}} = \frac{d - d_{0}}{d}$