With symbols having their usual meanings, which one among the following is used as the standard for defining atomic mass unit?

The correct answer is C-12.An Atomic Mass Unit is defined as accurately 1/12 the mass of a carbon-12 atom.Atomic mass is calculated by the sum of no. of Protons and Neutrons in the nucleus.The carbon-12 atom has six Neutrons and six Protons in its nucleus and thus represented as C12.The atomic unit mass is denoted as 'amu' and 1 amu = 1.67377 x 10-24 gram.Atomic no. of C12 atom is 6 and it belongs to the 2nd Period and 14th Group in the periodic table. Hydrogen:It is the first ELEMENT in the periodic table having atomic no. = 1 and mass no. = 1 amu.Hydrogen has three naturally occurring isotopes: H1 (Protium), H2 (Deuterium), and H3 (Tritium). It is the most abundant element in the universe.It is placed in 1st Period and1st Group.Nitrogen:It is the seventh element in the periodic table having atomic no. = 7 and mass no. = 14 amu.Nitrogen is the most abundant gas in the atmosphere, nearly 78%.It is placed in 2nd Period 15thGroup.Oxygen:It is the eighth element in the periodic table having atomic no. = 8 and mass no. = 16 amu.Oxygen is the second most abundant gas in the atmosphere, nearly 21%.It is placed in 2nd Period 16THGROUP.