How many sides does a regular polygon have whose interior and exterior angle are in the ratio 2 : 1 ?

Answer: Option C

Each exterior angle of n sided polygon is
$$\text{ = }\left(\frac{360}{n}\right)$$
And each interior angle of n sided polygon
$$\begin{array}{rl}& \text{ = }\frac{\left(n-2\right)\times 180}{n}\\ & \therefore \frac{\frac{\left(n-2\right)\times 180}{n}}{\frac{360}{n}}=\frac{2}{1}\\ & \Rightarrow \frac{\left(n-2\right)}{2}=2\\ & \Rightarrow n-2=4\\ & \Rightarrow n=6\end{array}$$