Assertion: If changing current is flowing through a machine with iron parts, results in loss of energy.
Reason: Changing magnetic flux through an area of the iron parts causes eddy currents.
A) ASSERTION and REASON are individually true and REASON is the correct explanation for ASSERTION.
B) ASSERTION and REASON are individually true but REASON is not the correct explanation for ASSERTION.
C) ASSERTION is correct but REASON is incorrect
D) Both ASSERTION and REASON are incorrect.

Hint: To solve this question we should try to understand the effects of a changing current flow that can be produced in a body. We can use the Faraday’s Law of Induction to understand this effect and its consequences.

Complete step by step answer:
We all know about Faraday’s law of Electromagnetic induction. It consists of two laws.
The first law states that whenever electric current flows through a conductor, a magnetic field is produced and conversely a changing magnetic field generates an electric current.
The second law states that the change in magnetic flux with time produces an electromotive force.
$\Phi = B.A = BA\cos \theta $
Where $\Phi $ is the flux produced, $B$ is the magnetic field, $A$ is the surface area and $\theta $ is the angle between the magnetic field lines and normal to surface area.
$\varepsilon = - N\dfrac{{d\Phi }}{{dt}}$
Where $\Phi $ is the flux produced, $t$ is the time, $N$is the number of turns in the coil and $\varepsilon $ is the electromotive force or the EMF produced.
In this case, the changing current generates a magnetic field. As the parts are made of iron, the flux will be different at different parts of the machine and so EMF will be produced. This EMF can produce eddy current which will produce heat and hence loss of energy in the form of heat.

So the reason is correct for the assertion. Hence the answer will be (A).

Note: Eddy currents are loops of electrical current in a conductor carrying changing electric current. The loops are perpendicular to the magnetic field. They are induced within conductors by a changing magnetic field in a conductor due to Faraday's law of induction. These mostly lead to loss of useful energy in the form of heat and hence should be tried.