by natural decay
Recall the following decay processes, and recall that the subscripts and superscripts have to balance on both sides of the arrow.
Beta () Decay:
where is the symbol of an anti-neutrino. (The decay is accompanied by a neutrino, , instead of an anti-neutrino.)
Gamma Decay (no change in A or Z):
Deuteron Decay (rare):
(A) . This works out.
(B) decay always occurs with an antineutrino.
(C) emission doesn't change the atomic number.
(D) Although the numbers work out here, deuteron decay is rare, thus not natural.
(E) The subscripts and superscripts don't add up.