Pu-238 (Plutonium, 238) decays by α emission to form an atom, which atom is this?

Helium atom

Explanation:

For the RTGs flown by the United States, the radioisotope has been some chemical form of plutonium-238 (238Pu or Pu-238). Plutonium-238 decays primarily by the emission of an alpha particle (a helium atom without its electrons).

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .

catya that's not even a b or c