How many kilometres are there in $1\text{mm}$?

We know the metric system relationships:
$$1\text{m}=1000\text{mm}$$
$$1\text{km}=1000\text{m}$$
Therefore:
$$1\text{km}=1000\times 1000\text{mm}=1,000,000\text{mm}$$
$$1\text{mm}=\frac{1}{1,000,000}\text{km}=10^{-6}\text{km}=0.000001\text{km}$$
Looking at the options, option C provides the correct decimal representation.