Which is the smallest unit of measurement?

The millimeter is the smallest unit of measurement among the options provided. It is part of the metric system, where lengths are measured in increments of ten. One millimeter equals one thousandth of a meter, making it significantly smaller than a meter, a kilometer, or a decimeter.

To contextualize further, a meter is the base unit for length in the metric system and serves as the standard measurement for larger distances. A kilometer is equal to 1,000 meters, representing a much larger scale typically used for distances between geographical locations. A decimeter is one-tenth of a meter, which is still larger than a millimeter, making it less precise for smaller measurements. Thus, understanding the hierarchy within the metric system clearly illustrates why the millimeter is the smallest unit of measurement in this context.