A nanometre (nm) is equal to one ___ of a metre.

Correct option is D

A nanometre (nm) is equal to one billionth (10⁻⁹) of a meter.
Explanation:
· (a) Hundredth – This is 10⁻² of a meter, not a nanometre.
· (b) Thousandth – This is 10⁻³ of a meter, which is a millimetre, not a nanometre.
· (c) Millionth – This is 10⁻⁶ of a meter, which is a micrometre.
· (d) Billionth – Correct answer. A nanometre is one billionth of a meter (10⁻⁹).