Final answer:
Radioactive decay is the spontaneous transformation of a nucleus emitting radiation, resulting in a change in the number of protons and/or neutrons and often leading to a different element, known as transmutation.
Step-by-step explanation:
When we say that a nucleus undergoes radioactive decay, we mean that it undergoes a spontaneous transformation, emitting radiation (Option C). This process often results in the nucleus transforming into a more stable configuration, which may involve a change in the number of protons and/or neutrons, leading to the creation of a different element, known as transmutation.
For example, in alpha decay, a nucleus emits an alpha particle (two protons and two neutrons), resulting in a decrease in both atomic mass and atomic number. In beta decay, a neutron is transformed into a proton and an electron, causing an increase in atomic number but no change in atomic mass.