(c)
Given equation is $a + bv^{2} = x^{2}$
On differentiating with respect to $t$, we get
$0 + b\left( 2v\frac{dv}{dt} \right) = 2x\frac{dx}{dt}$
$\Rightarrow \ \ \ vb\frac{dv}{dt} = x\frac{dx}{dt}$
$\Rightarrow \ \frac{dv}{dt} = \frac{x}{vb}.\frac{dx}{dt}$
$\Rightarrow \ \frac{dv}{dt} = \frac{x}{b}\left( \because\ \ \frac{dx}{dt} = v \right)$