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Questions asked by users might not always be phrased in the clearest way.
The correct answer is alpha.
The reaction is:
²³⁹Pu + α → ²⁴²Cm + ¹n
This is an example of alpha decay where a plutonium-239 nucleus absorbs an alpha particle (which is essentially a helium nucleus, ⁴He), resulting in the formation of curium-242 and a neutron.