SAT Question b85c19ed: Form Structure And Sense answer and explanation

English - Form Structure And Sense - Pronoun Antecedent Agreement
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The violins handmade in the seventeenth century by Italian craftsman Antonio Stradivari have been celebrated as some of the finest in the world. In close collaboration with musicians, Stradivari introduced changes to the shape of a traditional violin, flattening some of the instrument’s curves and making ______ lighter overall.
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Difficulty:Medium
Which choice completes the text so that it conforms to the conventions of Standard English?