(d)
Given, $y = a^{x}.b^{2x - 1}$
$\Rightarrow \log{y = x\log{a + (2x - 1)\log b}}\ \ \ \ \ \ \ \ \ \ \ \$
$\Rightarrow \frac{1}{y}.\frac{dy}{dx} = \log{a + 2\log b}$
$\Rightarrow \frac{dy}{dx} = y\log{ab^{2}}$
$\Rightarrow \frac{d^{2}y}{dx^{2}} = \frac{dy}{dx}\log{ab^{2} = y\ {(\log{ab^{2})}}^{2}}$