Final answer:
The correct answer is B. protons, transmutation, as the number of protons in an atom's nucleus determines its element and changes to this number result in a different element.
Step-by-step explanation:
When one element changes into another, the number of protons changes and the process is called transmutation. This is a type of nuclear reaction where the identity of an atom is changed because the nucleus, which contains the protons, has undergone a change. The number of protons in the nucleus determines the atomic number, which dictates the element to which an atom belongs. Radioactive elements naturally undergo this process, often emitting particles such as alpha and beta particles as well as gamma rays, thereby changing into a different element with a different number of protons. Transmutation can also be artificially induced, for instance, by bombarding a target with high-energy particles, which can produce a new element.