A radioactive isotope of hydrogen, \({ }^{3} \mathrm{H},\) is called tritium. Tritium differs from the more common form of hydrogen because (A) it contains two neutrons and one proton in its nucleus \((\mathrm{B})\) it contains one neutron and two protons in its nucleus (C) it differs by its atomic number (D) it is radioactive and therefore gives off one electron

An element is defined by its atomic number, which represents the number of protons in the nucleus. Hydrogen has an atomic number of 1, meaning it has one proton. However, hydrogen has different isotopes, which have the same number of protons but different numbers of neutrons. The most common isotope of hydrogen has no neutrons (protium), but tritium has two neutrons.

Now, we will evaluate each option to identify the correct answer. (A) it contains two neutrons and one proton in its nucleus: This option is correct. Tritium is an isotope of hydrogen with two neutrons and one proton in its nucleus, making it different from the more common form of hydrogen, which has no neutrons. (B) it contains one neutron and two protons in its nucleus: This option is incorrect. If it had two protons, it wouldn't be hydrogen–it would be helium. (C) it differs by its atomic number: This option is incorrect. Both tritium and common hydrogen have the same atomic number (1), which represents the number of protons in the nucleus. (D) it is radioactive and therefore gives off one electron: This option is partially correct. Tritium is radioactive, but radioactivity is not the primary difference between tritium and the more common form of hydrogen. Radioactivity is a byproduct of tritium's different nucleus structure, which contains two neutrons and one proton.

Based on the evaluation of the given options, option (A) is the correct answer. Tritium differs from the more common form of hydrogen because it contains two neutrons and one proton in its nucleus.