ANSWER
3) radium
EXPLANATION
For an element to spontaneously split into a lighter nucleus (and an even lighter helium nucleus), the element must be a radioactive element.
From the options, the only naturally radioactive element is radium which splits (decays radioactively) as follows:
[tex]Ra\to X+\alpha[/tex]where X = lighter nucleus, α = helium nucleus.
Therefore, the answer is option 3.
