The word that fills the blank has to characterize how the land grades into the ice in a way that explains how you can walk off the coast and over the sea without knowing it. The word that does that is "imperceptibly"; if the land grades imperceptibly into the ice, you might well not know that you had left the land. Describing the shift from land to ice as permanent, irregular or precarious would not help to explain how you would fail to know, while describing it as slight would suggest that there was hardly any grading from one to the other, the opposite of what is needed.
Thus the correct answer is Choice B (imperceptibly).
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