Given,

3942÷64÷3=?

It can be approximated as

= 3900÷60÷3=?

$$\begin{gathered} \text{=3900}\times \frac{1}{60}\times \frac{1}{3}= ? \\ \text{=}\frac{3900}{180}=21.66\approx 21 \end{gathered}$$

Hence, Option C is correct.