B. why is this number called a "mass” number?

The mass of the atom depends on the sub atomic particles present in the nucleus of an atom.

That is, the protons and the neutrons.

Electrons are present around the nucleus and the mass is negligible since its mass is very very less.

mass of a proton =

mass of a neutron =

mass of an electron =

Mass number represents the mass of one particular isotope and it is a whole number for example,

Mass number is 13 and atomic number is 6 for the carbon isotope C-13.  

Atomic mass is different from mass number and it is a fraction since it is the average atomic mass of all the isotopes of an atom.

Atomic mass of C is 12.011 amu which we see in the periodic table is the average atomic mass of isotopes C-12, C-13 and C-14.