Which one is not called an anode reaction from the following?
A. \[{\rm{C}}{{\rm{l}}^ - } \to \frac{1}{2}{\rm{C}}{{\rm{l}}_{\rm{2}}} + {e^ - }\]
B. \[{\rm{Cu}} \to {\rm{C}}{{\rm{u}}^{ + + }} + 2{e^ - }\]
C. \[{\rm{H}}{{\rm{g}}^ + } \to {\rm{H}}{{\rm{g}}^{ + + }} + {e^ - }\]
D. \[{\rm{Z}}{{\rm{n}}^{2 + }} + 2{e^ - } \to {\rm{Zn}}\]

Hint: There are two electrodes namely cathode and anode that consist of an electrochemical cell. These electrodes are placed in a solution called electrolytes and two half cells are formed. The combination of these two half cells leads to the formation of a cell.

Complete Step by Step Solution:
Let's understand what type of reactions occur at the cathode and anode. Anode is the electrode of an electrochemical cell where oxidation reaction occurs and at the cathode reduction reaction occurs.

Now, we will discuss oxidation and reduction. Oxidation is that reaction where loss of electrons occurs and reduction is the reaction in which gaining of electrons occurs.
Now, we have to identify the option where oxidation does not occur.

In option A, chloride ions lose one electron. So, it is an oxidation reaction. Therefore, this reaction takes place at anode.
In option B, copper losses two electrons. So, it is an oxidation reaction. Therefore, this reaction takes place at anode.
In option C, mercury ions lose one electron. So, this reaction is an oxidation reaction and occurs and anode.
In option D, zinc ions accept two electrons to form zinc metal. Therefore, this is a reduction reaction and thus occurs at cathode.
Hence, option D is right.

Note: The increase of oxidation number of an element is also termed oxidation and the decrease of oxidation number is termed reduction. The reaction where both oxidation and reduction happens simultaneously is termed redox reaction.