Particles can exist in classically forbidden regions outside the finite square-well potential.
The correct option is D.
In the absence of quantum effects, everything would be continuous, smooth, and completely predictable in the Newtonian sense. However, we may not be here to get bored with this if Planck's constant were 0.
At the atomic scale, the behavior of both particles and waves is described by Planck's constant. One of the factors that led to the creation of quantum mechanics is Planck's constant. It was a surprise when those boundaries also turned to represent the limits where known rules of physics applied since the Planck scaled was developed as an universal set of units.
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