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Explanation:

Option 4 succinctly summarizes the crux of the author’s arguments. The author is trying to propound the ‘instinctive’ nature of language. He mentions it is unique to “Homo sapiens” but that is not the crux of the passage making option 1 unsuitable as a summary.
Option 2 focuses on a non critical aspect of language which deals with ‘learning’ or ‘teaching’, which is true yet not central to the discussion.
Option 3 is incorrect as it focuses on ancillary aspects.
Option 5 has no base in the paragraph, and is a radical statement.
Hence, the correct answer is option 4.