Answer: c ) Different isotopes of hydrogen
Explanation: In chemistry, you define an isotope from one element, as the one composed by same number of protons, but different number of neutrons. This difference makes isotopes to have different physical properties
For Hydrogen, you may find 3 isotopes: Hydrogen (which has 1 proton and contains no neutrons), Deuterioum (which contains 1 proton and 1 neutron) and Tritium (which contains 1 proton and 2 neutrons)
Proton is referrenced as "p" and neutron is referrenced as "n"