The allusion is the passage is found in "To attack this mass of granite, even by mining, was a Herculean task, and it was truly vexing that nature had not helped them in their necessity." (option B)
Allusion happens when we mention a character belonging to another existing story but we do not offer any explanation about that character. We assume readers will already know, since we usually allude only to well-known characters.
In the passage, the allusion is found in the phrase "Herculean task," which compares the level of difficulty of something to the difficult tasks which Hercules, from Greek mythology, has to do.
Therefore, the author uses an allusion to Hercules in the passage. Notice he does not explain anything about the character, but assumes readers will understand the reference.
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