Final answer:
To properly punctuate the sentence, remove the comma after 'Thoreau'.
Step-by-step explanation:
To properly punctuate the sentence, the comma after 'Thoreau' should be removed. The sentence should read: 'Henry David Thoreau, often known as the father of environmentalism, wrote the book entitled Walden.' The comma is unnecessary because the phrase 'often known as the father of environmentalism' is an essential part of the sentence and should not be set off by commas.
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