ANS:A - atoms

An atom is the basic unit of a chemical element, consisting of a nucleus surrounded by one or more electrons. The nucleus contains protons and usually neutrons, except for hydrogen, which has no neutrons in its most common form. In the context of radioactive decay, each atom of a radioactive isotope has a nucleus that is unstable, meaning it has an excess of energy or an unstable ratio of protons to neutrons. As a result, these nuclei undergo spontaneous decay over time, emitting radiation in the process. The concept of half-life is particularly relevant when discussing radioactive decay. The half-life of a radioactive isotope is the time it takes for half of the atoms in a sample of the isotope to undergo radioactive decay. This means that if you start with a certain number of radioactive atoms, after one half-life, half of those atoms will have decayed into a different element or isotope, and the other half will remain. After another half-life, half of the remaining atoms will decay, leaving one-quarter of the original amount, and so on. So, when we say the half-life of a radioactive isotope corresponds to the time required for half of the atoms to decay, we mean that it describes the rate at which the unstable nuclei of the atoms in a sample undergo radioactive decay, leading to a reduction in the number of radioactive atoms over time.