Answer
(b) Linear
Work Step by Step
Having a quadratic function in its standard form: (Note: highest power of $x$=2) $$f(x)=Ax^2+Bx+C$$
Taking derivative of $f(x)$ with respect to $x$ using the power rule; $f'(x)=\frac{d }{dx}(x^n)=nx^{n-1}$
we get; $$f'(x)=2Ax+B$$
this result is a linear equation (highest exponent of $x$ is $1$) of the form $y=mx+c$, where, $m=2A$ is the gradient of the line.
The correct answer is $(b)$.
