The correct answer is C. Nm.
The ozone layer is a region of Earth’s stratosphere that absorbs most of the Sun’s ultraviolet radiation. It contains high concentrations of ozone (O3) relative to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth’s atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 20 to 30 kilometers (12 to 19 mi) above Earth, although its thickness varies seasonally and geographically.
The thickness of the ozone layer is measured in nanometers (nm). A nanometer is one billionth of a meter, or 0.000000001 meters. The ozone layer is typically about 300 nm thick, but it can vary depending on the time of year and the location.
The other options are incorrect. Millimeters (mm) are a unit of length equal to one thousandth of a meter. Decadal units (Du) are a unit of time equal to ten years. Mercury (Hg) is a chemical element with the symbol Hg and atomic number 80.