SAT Question 43b88827: Form Structure And Sense answer and explanation

English - Form Structure And Sense - Subject Verb Agreement
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In October of 1648, representatives from 96 different parties convened to sign the final treaties of the Peace of Westphalia. The treaties, which brought an end to the Thirty Years’ War, ______ signed in the cities of Osnabrück and Münster, both located in the Holy Roman Empire–controlled region of Westphalia.
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Difficulty:Medium
Which choice completes the text so that it conforms to the conventions of Standard English?