Neutrons is your answer my friend
It's B, because a negative charged Ion is an anion. And since it's 1- ,one valence electron is been added to H. So it has 2 valence electrons.
Explanation:
The given reaction equation is as follows.
+ ___ 
As there is release of one hydrogen which shows that mass number has increase by 1 on the product side.
Therefore, particle
must be added in order to balance the given reaction equation.
Hence, the complete reaction equation will be as follows.
