When plutonium-239 undergoes alpha decay, it becomes a) curium-241 + an alpha particle b) uranium-235 + an alpha particle c) neptunium-237 + and alpha particle d) plutonium-237 + and alpha particle

An alpha particle is composed of two protons and two electrons.

As a result, in alpha decay, the nucleus that decays loses two protons and two neutrons. So the mass number (239 in this case) decreases by 4.

239 - 4 = 235,

So the new nucleus must be B) Uranium-235, as it is the only one of the options that has an atomic mass of 235.