(c)
Given , $t = as^{2} + bs + c$
$\Rightarrow \ \ 1 = 2as\frac{ds}{dt} + b\frac{ds}{dt}$ [differentiating]
$\Rightarrow \ \ 1 = 2asv + bv$ …(i)
$\Rightarrow 0 = 2a\frac{ds}{dt}v + 2as\frac{dv}{dt} + b\frac{dv}{dt}\$ [differentiating]
$\Rightarrow \ \ \ \frac{dv}{dt}(2as + b) = - 2av^{2}$
$\Rightarrow \ \ \ \frac{dv}{dt}\left( \frac{1}{v} \right) = - 2av^{2}$ [from Eq.(i)]
$\Rightarrow \ \ \ \frac{dv}{dt} = - 2av^{3}$