What is the smallest unit of measurement in the metric system?

The smallest unit of measurement in the metric system among the provided options is the nanometer. The metric system is a decimal system based on powers of ten, and it includes several prefixes that denote different scales of measurement.

The millimeter is indeed a small unit, representing one-thousandth of a meter, but it is larger than both the micrometer and the nanometer. A micrometer, also known as a micron, is one-millionth of a meter and is smaller than a millimeter. The nanometer, being one-billionth of a meter, is the smallest unit in this list, making it suitable for measuring things at the atomic and molecular level.

Thus, when considering the comparative sizes of these metric units, the nanometer is the correct choice as it represents the finest level of measurement in the context of the options given.