SAT Question 8462b105: Nonlinear Functions answer and explanation with Desmos solution

Choice C is correct. It's given that the function $f$ gives the product of a number, $x$, and a number that is $91$ more than $x$. A number that is $91$ more than $x$ can be represented by the expression $x+91$. Therefore, $f$ can be defined by the equation $f(x)=x(x+91)$, or $f(x)=x^2+91x$.