(c)
Given differentiating equation is
$\left( 1 + 4\ \frac{dy}{dx} \right)^{\frac{2}{3}} = 4\ \frac{d^{2}y}{dx^{2}}$
$\Rightarrow$ $\left( 1 + 4\ \frac{dy}{dx} \right)^{2} = 4^{3}\left( \frac{d^{2}y}{dx^{2}} \right)^{3}$
Here, highest order is 2 and degree is 3.