Final answer:
C) Half-life
The half-life is the primary factor that determines the rate of decay for a radioactive isotope; it is a constant value that is independent of external conditions.
Step-by-step explanation:
The primary factor that determines the rate of decay for a radioactive isotope is its half-life. The half-life is the time required for half of the nuclei in any sample of a radioactive isotope to decay.
It is a constant for each specific isotope and is independent of external conditions such as temperature and pressure. Isotopes with shorter half-lives decay more rapidly, which means they undergo a greater number of radioactive decays per unit time compared to isotopes with longer half-lives.
Despite other factors that affect chemical reactions, such as temperature or concentration, these do not influence the half-life of a radioactive substance.