The correct answer is: All hydrogen atoms (and ions) have same number of protons (1 proton, to be more precise).
Explanation:
1. The normal hydrogen atom has 1 proton (in the nucleus), 1 electron and 0 neutrons.
2. The deuterium (an isotope of hydrogen atom) has 1 proton (in the nucleus), 1 electron and 1 neutron (in the nucleus).
3. The tritium (an isotope of hydrogen atom) has 1 proton (in the nucleus), 1 electron and 2 neutrons (in the nucleus).
4. The hydrogen ion has 1 proton (in the nucleus), 0 electrons and 0 (or more) neutrons (depending upon the isotopes/normal atom).
As you can see in the above list that the only thing common in all is 1 proton (answer).
