Learning plateau is

A learning curve traces how a learner's proficiency changes with continued practice; it commonly shows phases of rapid gain followed by a plateau, a stretch where visible progress flattens out temporarily even though the learner keeps practising, before the curve typically resumes its climb.

Here, that flattening of visible progress despite continued effort is exactly what a plateau means, so it is described as an arrested growth in learning, growth that is temporarily halted rather than reversed or permanently blocked.

Contrast with the other descriptions on offer:

• “defect in learning” would mean an inherent flaw in the learner or the method; a plateau is a normal, near-universal stage of the learning curve, not a malfunction.

• “cessation of learning” would mean the curve stops permanently; a plateau is only a flat stretch, and the curve usually resumes rising once it ends, so learning has not stopped for good.

• "obstruction in learning" names a blocking barrier that stops progress outright; a plateau, by contrast, is simply the descriptive shape of the curve at that stage, a temporary flat stretch in measured progress, not a claim about what is blocking it.

That is why the flattening stage on the learning curve is best described as arrested growth in learning.