This phenomenon demonstrates electromagnetic damping, which occurs due to electromagnetic induction and Lenz's Law.

Step 1: Understanding the Setup
A coil carrying current is suspended in a uniform magnetic field with its plane parallel to the magnetic field lines. The magnetic field exerts a torque on the current-carrying coil, causing it to rotate. In this specific orientation, the coil experiences no net torque when stationary, but once displaced and with current flowing, it begins to oscillate like a torsional pendulum.

Step 2: Why it's Difficult to Stop
The oscillating coil has inertia. In the absence of any significant damping force (like air resistance or friction at the suspension point), its mechanical energy is conserved, and it continues to oscillate for a long time.

Step 3: The Role of the Aluminium Plate
Aluminium is a non-ferromagnetic (specifically, paramagnetic) metal, but it is an excellent electrical conductor. When the oscillating coil moves near this stationary conductor, the changing magnetic field from the coil induces eddy currents within the aluminium plate.

Step 4: Lenz's Law and Damping
According to Faraday's Law of electromagnetic induction, a changing magnetic flux induces an electromotive force (EMF). Lenz's Law states that the direction of this induced current will be such that it opposes the change in flux that produced it.
The induced eddy currents in the aluminium plate create their own magnetic field. This induced magnetic field always opposes the motion of the original coil. This opposing interaction acts as a braking force, rapidly dissipating the coil's kinetic energy as heat (Joule heating) within the plate. This process is called electromagnetic damping.

Final Answer: The correct explanation is electromagnetic induction in the aluminium plate giving rise to electromagnetic damping.

Related Topics & Formulae

Faraday's Law of Induction:
The induced EMF in a circuit is equal to the negative rate of change of magnetic flux through the circuit. $\epsilon =-\frac{d\Phi }{dt}$ where $\Phi$ is the magnetic flux.

Lenz's Law:
This is the physical law underlying the negative sign in Faraday's law. It determines the direction of the induced EMF and current, ensuring they oppose the change in magnetic flux that produced them.

Magnetic Flux:
The magnetic flux through a loop is given by: $\Phi =B\cdot A\cdot \cos \theta$ where $B$ is the magnetic field strength, $A$ is the area of the loop, and $\theta$ is the angle between the field and the normal to the loop's plane.