Assertion: Oxidation involves loss of electrons and reduction involves gain of electrons.
Reason: The overall reaction in which oxidation and reduction occurs simultaneously is called redox reaction.
A. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Assertion is incorrect but Reason is correct.

Hint: Oxidation as the name suggests is addition of oxygen and reduction is thus loss of oxygen. It can also be explained in terms of addition or removal of hydrogen or addition or release of electrons.

Complete step by step answer:
 Oxidation and reduction are described in various ways. In this case the oxidation and reduction are described in terms of transfer of electrons.
Oxidation and reduction reaction in terms of electron transfer are indicated as:
Oxidation is loss of electrons i.e. \[M \to {M^ + } + {e^ - }\] where \[M\] is a metal ion undergoing oxidation by release of electrons to form \[{M^ + }\] ion.
Reduction is gain of electrons i.e. ${L^ + } + {e^ - } \to L$, where \[L\] is another atom or molecule undergoing reduction by adding electrons to \[{L^ + }\] ion.
The simultaneous oxidation and reduction is explained with the help of an example. Let us consider a redox reaction between \[CuO\] and \[Mg\] metal. The corresponding reaction and ionic reaction is:
$CuO + Mg \to Cu + MgO$
$C{u^{2 + }} + M{g^0} \to C{u^0} + M{g^{2 + }}$
In this reaction, the copper is undergoing reduction by gain of electrons or loss of oxygen and the magnesium is undergoing oxidation by loss of electrons and gain of oxygen. Copper is going from \[ + 2\] to \[0\] oxidation state and magnesium is going from \[0\] to \[ + 2\] oxidation state.
Hence both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
So, the correct answer is “Option A”.

Note:
Oxidation and reduction can also be described in terms of hydrogen transfer. The oxidation is termed as loss of hydrogen and reduction is gain of hydrogen.