An oxidation process involves-
A. Increase in oxidation number
B. Decrease in oxidation number
C. Addition of elements.
D. Removal of electronegative element

Hint: Oxidation number is the charge which an atom appears to have in its ion form or when present in the combined state with other atoms While electro-negativity is the measure of tendency of an element to attract shared electrons towards itself in a covalently bonded molecule.

Step-by-Step Solution:
According to electronic concept-An Oxidation process is loss of electrons of an element or compound. Due to this there is an increase in positive charge or decrease in negative charge of that element’s atom. For example-
$Na \to N{a^ + } + {e^ - }$
In this example sodium loses its electron to convert into its ion. There is an increase in positive charge.
$C{l^ - } \to Cl + {e^ - }$
In this example, chlorine ion loses an electron and there is a decrease in negative charge.
According to classical concept- oxidation process is the addition of oxygen or any other electronegative element. It can also be defined as the process of removal of electropositive elements. For Ex-
$Zn + S \to ZnS$
In this reaction sulphur which is an electronegative element is added to zinc.
$2KI + C{l_2} \to 2KCl + {I_2}$
In this reaction the electro-positive element potassium is removed by chlorine molecules.
In terms of oxidation number- oxidation process in a chemical change in which there is an increase in oxidation number. The decrease in oxidation number is known as reduction. For ex-
\[\mathop {Zn}\limits^0 + 2\mathop H\limits^{ + 1} \mathop {Cl}\limits^{ - 1} \to \mathop {Zn}\limits^{ + 2} \mathop {C{l_2}}\limits^{ - 1} + \mathop {{H_2}}\limits^0 \]
The numbers above the elements show their oxidation number.
In this reaction oxidation number of zinc increases from $0$to$2$ so it is oxidized while the oxidation number of hydrogen decreases from $ + 1$to $0$ as hydrogen changes to its elementary state. So hydrogen is reduced while the oxidation number of chlorine does not change.
Oxidation and reduction reactions are complementary reactions. They always occur simultaneously like in the above reaction.

Hence the correct answer is A.

Note: Some important concepts-
- The element whose oxidation number decreases is known as oxidizing agent. It increases the oxidation number of other substances present in the chemical reaction. Like in the above reaction of zinc and hydrochloric acid, the acid is an oxidizing agent.
- The element whose oxidation number increases is known as a reducing agent. It decreases the oxidation number of other substances present in the chemical reaction. Like in the above reaction of zinc and hydrochloric acid, zinc is a reducing agent.