The differences are <u>the number of neutrons</u> and the <u>atomic mass</u><u /><u />.
Copper-63 has an atomic mass of 63 amu, and has 34 neutrons.
Copper-65 has an atomic mass of 65 amu, and has 36 neutrons.
I believe the correct answer from the choices listed above is option C. Electrons are both gained and <span>lost during a redox reaction. From the name itself "redox" or reduction-oxidation, it must be that something has reduced and another has gained that something.</span>