SAT Question 7bc05fa2: Words In Context answer and explanation

English - Words In Context - Fill In Blank Single Word
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Whether the reign of a French monarch such as Hugh Capet or Henry I was historically consequential or relatively uneventful, its trajectory was shaped by questions of legitimacy and therefore cannot be understood without a corollary understanding of the factors that allowed the monarch to ______ his right to hold the throne.
115
Difficulty:Hard
Which choice completes the text with the most logical and precise word or phrase?