A refrigerator is:
A) Heat engine
B) An electric motor
C) Heat engine working in backward direction
D) Air cooler

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Hint: Just keep in mind that the refrigerator generally operates on a principle in which heat transfer occurs in the opposite direction. In this process, heat is transferred from the cold reservoir to the hot reservoir. We can say that the refrigerator warms the air at room temperature.

Complete step by step solution:
As we know, a heat engine is something that converts heat into mechanical work.
Usually, a refrigerator is a device that is used to cool things.
But according to the second law of thermodynamics, a refrigerator is a device which is used to turn mechanical work into heat.
Therefore, we can say that a refrigerator is a heat engine that works in a backward direction.

Therefore, option (C) is the correct option.

Additional Information:
Now let us know the working principle of the refrigerator. Basically, a refrigerator consists of five components which are a fluid refrigerant, a compressor, a condenser coil, an evaporator coil, and an expansion device. The compressor is something that compresses the refrigerant vapor by raising its pressure and pushes it to the coil that is used in the refrigerator. When the gas in the coil meets the air at room temperature in the kitchen, it will become liquid. Now, in the liquid form, when it passes from the freezer that contains refrigerants, the liquid will cool down at high pressure. The refrigerant absorbs the heat present in the fridge and cools down it. The refrigerator then evaporates and flows back to the compressor, as a result, this process itself.

The coefficient of performance (COP) is defined as the ratio of the heat removed from the cold reservoir inside the refrigerator, which is represented as $${Q_{cold}}$$, to the work done by the compressor to remove the heat (W). The coefficient of the refrigerator is given by
$COP = \dfrac{{{Q_{cold}}}}{W}$
From the above expression, we can say that the more heat evolved inside the refrigerator the more efficient it will be.
As we know, according to the law of thermodynamics,
${Q_{cold}} + W = {Q_{hot}}$
$ \Rightarrow \,W = {Q_{hot}} - {Q_{cold}}$
Putting this value of work done in the above equation, we get
$COP = \dfrac{{{Q_{cold}}}}{{{Q_{hot}} - {Q_{cold}}}}$
Which is the equation of the coefficient of performance of the refrigerator.

Note: The above formula is also applicable for an air conditioner, which just acts like a refrigerator. On the other hand, the coefficient of performance of heating and cooling are different from each other. The coefficient of performance is highly dependent on the absolute temperature and relative temperature between the heat sink and the system.