The punctuation mark that can be used to replace the em-dash in this sentence is a comma because the function is to add a description.
This is a punctuation mark that is often used to add a sentence that complements or explains a previous one. Moreover, the em-dash can work in a similar way to:
Considering in the sentence the author is adding another clause that connects to the first, the best punctuation mark is a comma, which is often used to separate clauses, specially before words such as but.
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