Answer :
The spontaneous decay of an atom is known as transmutation.
• The conversion of one chemical element or an isotope into another chemical element is termed as nuclear transmutation.
• It takes place in any process where the number of neutrons or protons in the nucleus of an atom is changed.
• It is the spontaneous decay of an atom.
• Transmutation can be attained either by radioactive decay or by nuclear reactions. In radioactive decay no external cause is required, while in nuclear reactions an external particle reacts with a nucleus.
• One kind of natural transmutation is witnessed when some kind of radioactive elements found in nature spontaneously decay by a procedure that leads to transmutation like beta or alpha decay.
Thus, the correct answer is option 4, that is, transmutation.
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