The mass of an atom, measured in atomic mass units, is
a. exactly equal to the atomic mass number
b. always very close to the atomic mass number. a but could be a little more or a little less.
c. always slightly more than the atomic mass number a
d. always slightly less than the atomic mass number a.

The mass of an atom, measured in atomic mass units, is always very close to the atomic mass number. A but could be a little more or a little less.

Option B

Explanation:

Atoms are the most elementary or the building blocks of every object. So each atom consists of mass and atomic number. The mass number is the measure of total number of electrons and protons in the atom while the atomic number is the measure of number of electrons in the atom. Usually the mass number determine the mass of an atom.

But there is a slight difference, the mass number is denoted in amu units while the mass of the atoms are denoted in SI units of kg. So the amu unit is the atomic mass unit which is a measure of mass of any atom relative to the 12 atoms of carbon. So, mostly the atomic mass number in amu will be very close to the atomic mass number but it could be a little more or a little less than the mass of that atom.