Reduction is a process in which
(this question has multiple correct option)
A.Gain of electrons takes place
B.Oxidation number is reduced
C.Oxidation number is increased
D.Loss of electrons take place

Hint: Reduction is the opposite of Oxidation. Oxidation is the process in which there is the loss of hydrogen and Reduction is the process in which there is the gain of hydrogen.

Complete step by step answer:
The word reduction comes from Latin word which means ‘to lead back’. Reduction is a chemical reaction in which hydrogen is added to, or oxygen is removed from, in a compound.
So, in the reduction, there is loss of Oxygen and gain of hydrogen.
 \[C{H_3}CHO \to C{H_3}C{H_2}OH\]
In the above example, we see that Ethanal is reduced to Ethanol. There is a reduction by the addition of hydrogen to it.
Let’s take one more example
 \[MgO\left( s \right) + C\left( s \right) \to Mg\left( s \right) + CO\left( g \right)\]
The reaction between Magnesium Oxide (MgO) and Carbon (C) to form magnesium metal (Mg) and Carbon monoxide (CO). Here, MgO is reduced to Mg.
In reduction there is gain of electrons. The number of electrons that are gained or loss in a chemical reaction is the oxidation number of the element. When electrons are lost, oxidation number is increased but when the electrons are gained, the oxidation number is reduced. It increases the negative charge of a chemical species.
So, Reduction is the process in which gain of electrons takes place and oxidation number is reduced.

Therefore, the correct answers are option (A) and (B).

Note: And opposite to oxidising agents are reducing agents which reduces something. Reducing agents remove oxygen from another substance, it loses electrons and is oxidised in a chemical reaction. Examples of reducing agents can be earth metals, formic acid etc.