The area of a square whose side is $20 \mathrm{cm}$ is:

Given:

The side of a square is $20 \mathrm{cm}$.

Area of square $=\mathrm{Side} \times \mathrm{Side}$.

So, the area of the given square will be

$$\begin{gathered} =20\times 20 \\ =400 \mathrm{sq}. \mathrm{cm} \end{gathered}$$

Hence, option C is correct.