Which of the following represents the smallest unit of measurement?

The smallest unit of measurement among the options provided is nano. In the metric system, each prefix represents a different factor of ten. The prefix "nano" denotes one-billionth (10^-9) of a unit, making it significantly smaller than the other options listed.

When we consider the other prefixes:

• "Milli" represents one-thousandth (10^-3) of a unit.
• "Centi" stands for one-hundredth (10^-2) of a unit.
• "Micro" indicates one-millionth (10^-6) of a unit.

Thus, when comparing these prefixes, nano is the smallest because it is several orders of magnitude smaller than milli, centi, and micro. This distinction is important in fields such as sonography, where precision at a microscopic level may matter in measurements such as wavelengths or the size of structures being imaged.