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

C-12

The correct answer is option 1. C-12.

The atomic mass unit (amu) is a unit of mass used to express atomic and molecular weights. It is defined based on the mass of an atom of carbon-12 (\( \text{C-12} \)), which is chosen as the reference standard.

One atomic mass unit is defined as one twelfth of the mass of a neutral atom of carbon-12. The reason for choosing carbon-12 is that it is abundant, and its atomic nucleus has a well-defined structure with six protons and six neutrons.

So, by definition:

\[ 1 \, \text{amu} = \frac{1}{12} \text{th of the mass of a carbon-12 atom} \]

This standard allows scientists to compare the masses of different atoms and molecules on a common scale, making it a convenient reference point in the field of atomic and molecular mass measurements.