SAT Question 93724cc6: Percentages answer and explanation with Desmos solution

Choice D is correct. Let $x$ represent the number that $21$ is $21\%$ of. It follows that $\frac{21}{x}=\frac{21}{100}$. Multiplying each side of this equation by $x$ yields $21=\frac{21x}{100}$. Multiplying each side of this equation by $100$ yields $2,100=21x$. Dividing each side of this equation by $21$ yields $100=x$. Therefore, $21$ is $21\%$ of $100$.