Answer:
A. 35 neutrons
Step-by-step explanation:
The atomic number of copper is 29. That means it has 29 protons in the nucleus. Since electrons are too small to add significant mass, the only other particle that contributes to mass is the neutron. It has the same mass as the proton (1 amu each). 64Cu is telling us the atom of copper has an atomic mass of 64. Subtract the atomic mass of the 29 protons, at 1 amu each, we get (64-29) = 35 neutrons.