SAT Question e41dfaab: Words In Context answer and explanation

English - Words In Context - Fill In Blank Single Word
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In 1929 the Atlantic Monthly published several articles based on newly discovered letters allegedly exchanged between President Abraham Lincoln and a woman named Ann Rutledge. Historians were unable to ______ the authenticity of the letters, however, and quickly dismissed them as a hoax.
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Difficulty:Easy
Which choice completes the text with the most logical and precise word or phrase?