The least number among $\frac{4}{9}, \sqrt{\frac{9}{49}}, 0.\overline{45}$ and $\left ( 0.8 \right )^{2}$ is :
[A] $0.\overline{45}$
[B] $\left ( 0.8 \right )^{2}$
[C] $\frac{4}{9}$
[D] $\sqrt{\frac{9}{49}}$