One nanometre is equal to

The correct option is 10���⁹ m.

To understand why one nanometre is equal to 10⁻⁹ m, let us look at the definition of metric prefixes.
The prefix "nano-" is a standard unit prefix in the metric system that denotes a factor of 10⁻⁹.
Therefore, by definition:
1 nanometre (nm) = 10⁻⁹ metres (m)

Let us also verify the other options to see why they are incorrect:
1. 10⁹ mm is a very large distance (1,000,000,000 mm, which is equal to 1,000 km).
2. Since 1 m = 100 cm = 10² cm, we can convert nanometres to centimetres:
$1 \mathrm{nm}={\mathrm{10}}^{-9} m={\mathrm{10}}^{-9}\times {\mathrm{10}}^2 \mathrm{cm}={\mathrm{10}}^{-7} \mathrm{cm}$
Thus, 1 nanometre is equal to 10⁻⁷ cm (which makes "10⁻⁶ cm" incorrect).