21 is 21 % of what number?

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 $\mathrm{2,100}=21x$. Dividing each side of this equation by $21$ yields $100=x$. Therefore, $21$ is $21\%$ of $100$.

Choice A is incorrect. $21\%$ of $0$ is $0$, not $21$.

Choice B is incorrect. $21\%$ of $1$ is $0.21$, not $21$.

Choice C is incorrect. $21\%$ of $42$ is $8.82$, not $21$.