Because she knew many of the leaders of colonial America and the American Revolution personally, Mercy Otis Warren was continually at or near the center of political events from 1765 to 1789, a vantage point combining with her talent for writing to make her one of the most valuable historians of the era.
(B) a vantage point, when combined with her talent for writing, that made Unnecessary change in meaning. "when" implies a condition - only on combining with talent did the vantage point make her valuable. Eliminate.
(C) a vantage point that combined with her talent for writing, and it made "it" has no antecedent. Eliminate.
(E) and this vantage point, combined with her talent for writing, made Correct answer.