SAT Question fa36d803: Boundaries answer and explanation

English - Boundaries - No Punctuation Within Clause
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Long attributed to Jacques-Louis David, the preeminent Neoclassical painter of his day, the 1801 painting Marie Joséphine Charlotte du Val d’Ognes gained fresh attention in the 1990s when art historians discovered that the painting—which depicts a solitary young woman sketching—was actually the work of little-known French portrait ______ Marie-Denise Villers (1774–1821).
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Difficulty:Hard
Which choice completes the text so that it conforms to the conventions of Standard English?