Answer
Quadratic
Work Step by Step
Let $f(x)=a x^{3}+b x^{2}+c x+d$ is cubic function where
$a, b, c$ and $d$ are constant and $a \neq 0$
Since
\begin{align*}
f'(x)&=\frac{d}{dx}(a x^{3}+b x^{2}+c x+d)\\
&=3ax^2+2bx+c,\ \ \ a\neq 0
\end{align*}
Then $f^{\prime}(x)$ is a quadratic function. Hence derivative function of any third degree function will be a second degree
function. i.e a quadratic function.
