Atlantic Ocean has a deeply/deep indented coast line which facilitates
trading and other mercantile activities.
My understanding says deeply would be the appropriate choice. But I'm not sure.
Learn English – Deep vs Deeply in the sentence
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Best Answer
They mean two different things; which one you want depends on which meaning you want.
As a side note, it should be written as The Atlantic Ocean rather than just Atlantic Ocean
This indicates a greater magnitude of indentation of the coast line.
This would likely be written with a comma (deep, indeted coast line), and means that the coast line is both deep and indented.