One hundredth of centimetre when written in fractions of kilometres, is equal to :

Answer: Option A

Required fraction :
$$\begin{array}{rl}& =\frac{\frac{1}{100}\text{ cm}}{1\text{ km}}\\ & =\frac{\left(\frac{1}{100}\right)\text{ cm}}{\left(1000\times 100\right)\text{ cm}}\\ & =\frac{1}{100\times 1000\times 100}\\ & =\frac{1}{10000000}\\ & =0.0000001\end{array}$$