Explanation

(c) Let L be the length of each side of cube. Initial volume =${\ L}^{3}.\ $When each side decreases by 1%.
New length $L^{'} = \ L - \frac{1}{100} = \frac{99L}{100}\ $
New volume = $L^{'3} = \left( \frac{99L}{100} \right)^{3}\ ,\ $change in volume,
$\mathrm{\Delta}V = L^{3} - \ \left( \frac{99L}{100} \right)^{3}$
= $L^{3}\left\lbrack 1 - \left( 1 - \frac{3}{100} + \cdots \right) \right\rbrack = \ L^{3}\left\lbrack \frac{3}{100} \right\rbrack = \frac{3L^{3}}{100}$
$\therefore Bulk\ strain = \frac{\mathrm{\Delta}V}{V} = \ \frac{3L^{3}/100}{L^{3}} = 0.03$