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Answer = B = Neutrons and Mass Number
Isotopes are defined as those atoms which have same atomic number but different atomic masses.
Atomic mass is basically the number of protons and neutrons present in an atom.
Atomic number is the number of protons present in an atom.
So, in isotopes the number of protons are same but the number of neutrons vary due to which atomic masses also vary.
In given three isotopes, all have same number of protons but different number of neutrons.
i.e.
H-1 = 1 P + 0 N = 1 u (Proton)
H-2 = 1 P + 1 N = 2 u (Deuterium)
H-3 = 1 P + 2 N = 3 u (Tritium)
Hence, it is clear that the number after H shows a change in number of neutrons and mass number.
The change of state that has been given in the question is called sublimation. It is actually the process in which a solid gets directly converted to gas without changing into liquid. I hope that this is the answer that you were looking for and the answer has actually come to your desired help.