Assertion: Insulators do not allow flow of current through them
Reason: Insulators have no charge carriers.
${\text{A}}{\text{.}}$ Both assertion and reason are correct and reason is the correct explanation for assertion.
${\text{B}}{\text{.}}$ Both assertion and reason are correct but reason is not the correct explanation for assertion.
${\text{C}}{\text{.}}$ Assertion is correct but reason is incorrect.
${\text{D}}{\text{.}}$ Both assertion and reason are incorrect.

Hint:A statement and an explanation about that particular statement are called assertion and reason.
The given question is called a stem and the options are called alternatives.
The reason is making any sense towards assertion.

Complete step by step answer:
Insulators do not have free charge carriers which will flow so they are bad conductors and the insulators do not allow the flow of current.
-If both assertion and reason are true and reason is the correct explanation of assertion.
-If both assertion and reason are true but reason is not the correct explanation of assertion.
-If assertion is true but reason is false.
-If assertion and reason both are false.
Hence, the correct answer is option \[\left( A \right)\]

Additional information:
When it involves the flow of electrical current, all materials constitute two categories:
-Conductor
-Insulators
The conductors of electricity are Materials which allow electric current to pass through them. These materials are made up of atoms whose electrons can move away freely. Insulators are called any material that does not allow electric current to pass through.
An electric insulator might be very little electric current and it flows through it because it should be under the influence of any electric field.

Note:There is \[10\] electrical insulators are followings,
-Rubber.
-Glass.
-Pure water.
-Oil.
-Air.
-Diamond.
-Dry wood.
-Dry cotton.