Which of the following is NOT an abbreviation for a volume measurement?

The choice of "mg" is correct because it stands for milligram, which is a unit of mass, not volume. In the context of medication and dosage, volume measurements typically refer to how much liquid a substance occupies, while mass measurements describe the amount of substance in terms of weight. On the other hand, "cc" refers to cubic centimeters, which is a direct volume measurement; "gtt" stands for drop, a common measure in dispensing liquids; and "gm," or gram, while primarily a mass measurement, can be misunderstood as volume depending on the context of specific densities. However, it is still mass. Therefore, "mg" stands out distinctly as not being a measurement of volume.