Which particle, when emitted from the nucleus of an atom, would have the greatest effect on the mass of the atom?
Consider the following.
The nucleus contains only protons and neutrons, so we would have to compare the mass of both of them.
As you can tell, a proton has much more mass than an electron, so it should have the greater effect on the mass of the atom.
However, emitting a proton does change the element of the atom, so I am not certain of which one actually affects the mass the greatest. It may possibly depend on the element of the atom itself!