The correct answer is A. Isotopes.
Isotopes are atoms of the same element that have different numbers of neutrons. They have the same number of protons, which determines the element’s atomic number, but different numbers of neutrons, which determines the element’s mass number.
For example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons. Both are isotopes of carbon, but carbon-14 is radioactive.
Cations are atoms that have lost one or more electrons, giving them a positive charge. Anions are atoms that have gained one or more electrons, giving them a negative charge. The Higgs boson is a subatomic particle that is thought to give other particles mass.
Isotopes are important because they can be used to trace the movement of elements through the environment. They are also used in medical imaging and other applications.