The correct answer is: $0.001\ \text{mol}$
Explanation
The prefix milli- means one thousandth, or $10^{-3}$. So one millimole is one thousandth of a mole.
Steps:
- Definition of milli: $1\ \text{milli} = 10^{-3}$
- Convert millimoles to moles: $1\ \text{mmol} = 1\times10^{-3}\ \text{mol}$
- Decimal form: $1\ \text{mmol} = 0.001\ \text{mol}$
Therefore, a millimole contains $0.001$ moles.