The isotopes of hydrogen are:

We begin by recalling the basic definition of an isotope. Two or more atoms are called isotopes of the same element when they possess the same atomic number $$Z$$ (same number of protons, thus belonging to the same element) but different mass numbers $$A$$ because their nuclei contain different numbers of neutrons.

Hydrogen has atomic number $$Z = 1$$, meaning every hydrogen nucleus must contain exactly one proton. However, the number of neutrons present in the nucleus can vary, and this leads to three distinct nuclides (nuclear species):

1. We have the nuclide with one proton and zero neutrons. Its mass number is

$$A = Z + \text{(number of neutrons)} = 1 + 0 = 1.$$

This isotope is written as $$^1_1\text{H}$$ and is traditionally called protium.

2. Next, we consider the nuclide that again has one proton but now possesses one neutron. Its mass number is

$$A = 1 + 1 = 2.$$

This isotope is written as $$^2_1\text{H}$$ and is called deuterium.

3. Finally, there is the nuclide with one proton and two neutrons. Its mass number becomes

$$A = 1 + 2 = 3.$$

This isotope is written as $$^3_1\text{H}$$ and is called tritium.

Thus, hydrogen naturally exists in three isotopic forms: protium, deuterium and tritium. No other combinations of neutron numbers are stable or commonly listed for hydrogen in introductory chemistry.

Looking at the options, only Option A lists all three—protium, deuterium and tritium—together, matching exactly with the set we have just derived.

Hence, the correct answer is Option A.