The square root of 0 . 4 is ?

Answer: Option A

$$\begin{array}{rl}& =\sqrt{\text{0}\text{.}\overline{\text{4}}}\\ & =\sqrt{\frac{4}{9}}\\ & =\frac{2}{3}\\ & =\mathrm{0.666......}\\ & =0.\overline{6}\end{array}$$