SAT Question c4259674: Nonlinear Functions answer and explanation with Desmos solution

Choice A is incorrect. This is the value of $f\left(21\right)$ when $f\left(x\right)=-4x$, rather than $f\left(x\right)=4x^{-1}$.

Choice B is incorrect. This is the value of $f\left(21\right)$ when $f\left(x\right)={\left(4x\right)}^{-1}$, rather than $f\left(x\right)=4x^{-1}$.

Choice D is incorrect. This is the value of $f\left(21\right)$ when $f\left(x\right)=\left(4^{-1}\right)x$, rather than $f\left(x\right)=4x^{-1}$.