The protons in an nucleus of an atom will not change unless a nuclear reaction takes place. The number of protons is equal to the atomic number of the element. For a neutral atom, the number of electrons and protons are equal. When they are unequal, then the atom occurs as an ion. It will has a net charge with it. The ion O²⁻ has a net charge of negative positive 2 because it has 2 more electrons than its protons. Since neutral oxygen has 8 protons, then O²⁻ ion has 8 protons and 10 electrons.
The reaction of HCl and NaOH is HCl + NaOH = NaCl + H2O. So the mole number of HCl and NaOH is equal. So the volume of HCl =0.01*0.1/0.02=0.05 L =50 ml. So the answer is D).