What does the atomic mass describe?
a. the mass of the protons in the atom
b. the mass of the nucleus of the atom
c. the mass of the neutrons in the atom
d. the mass of the electrons in the atom

B., the mass of the nucleus of the atom.
Both the protons and the neutrons are inside the nucleus. The atomic number of an atom is the number of protons, and if you subtract the atomic number from the atomic mass you can find the number of neutrons an atom has. 
So, the mass of the protons plus the mass of the neutrons, which are both inside the nucleus, equals the atomic mass. 

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