The nucleus of the isotope tritium contains one proton and two neutrons, and no electrons. This means that the correct option is "two neutrons and one proton." The atomic number of an element is determined by the number of protons in the nucleus. Since tritium has one proton, its atomic number is 1, and it is a hydrogen isotope. The mass number of tritium is 3, which is the sum of its protons and neutrons.