A stock solution is a solution of a known concentration. Stock solution has a high concentration and therefore, the known amount of stock solution is used for preparing different concentrations, by diluting the same with the known amount of the solvent being used, such as water.
Answer : The correct option is, 
Explanation :
- Oxidation reaction : It is defined as the reaction in which a substance looses its electrons. That means, the loss of electrons takes place.
Or we can say that, oxidation reaction occurs when a reactant losses electrons in the reaction.
- Reduction reaction : It is defined as the reaction in which a substance gains electrons. That means, the gain of electrons takes place.
Or we can say that, reduction reaction occurs when a reactant gains electrons in the reaction.
According to the electrochemical series,
most likely to be reduced because
Hence, the ion most likely to be reduced is
.