Explain why the mole is used as a counting unit for mass

It isn't.  

Explanation:

It is a counting unit for "things" — atoms, molecules, ions, etc. —  and these things all have different masses.

For example, a chemist may know that one atom of Cu reacts with one atom of S to form one molecule of CuS.

Cu + S ⟶ CuS

She knows that 1 mol of Cu (6.022 × 10²³ atoms of Cu) has a mass of 63.55 g and 1 mol of S (6.022 × 10²³ atoms of S) has a mass of 32.06 g.

By measuring  out 63.55 g of Cu and 32.06 g of S, she knows that there are just enough atoms of each element to react with each other.

Thus, the mole has enabled her to count atoms.

i would say b

explanation:

but i think i would need more information than that

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.