Answer: Redox Reaction
Explanation:
Redox reaction is the key chemical events in an oxidation-reduction also called Redox. It is the net movement of electrons from one reactants to another
Bromine (Br) is the only element listed that is a diatomic element. I believe that Bromine is going to be your answer.
"A neutron turns into a proton and an electron
Nominal mass does not change
atomic number decreases by one" -redbeardthegiant
The answer is D the oldest layer