The S.I. base unit of mass is the kilogram (kg), which has evolved from being defined by a physical object to a value based on fundamental constants. The correct answer to the student's question is option 3) kg.
The S.I. base unit of mass is the kilogram (kg), which is the only SI base unit with a prefix. Initially, the kilogram was defined by the mass of a platinum-iridium alloy cylinder housed at the International Bureau of Weights and Measures, but this definition changed in May 2019.
It is now based on fundamental physical constants. One kilogram is approximately equal to 2.2 pounds, and other common units such as the gram (g) and milligram (mg) are derived from the kilogram, with 1 gram being 1/1000 of a kilogram and 1 milligram being 1/1000 of a gram.
In conclusion, answer option 3) kg is correct for the S.I. base unit of mass.