This question is testing MEANING, logical MODIFIERS, and CLAUSE structure.

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.

(A) a vantage point combining with her talent for writing to make
- "combining" is an illogical/awkward modifier for "vantage point." Here, we're talking about 2 advantages that together made her a valuable historian: vantage point + talent. The past participle "combined" would make sense: these two things added together. The ongoing meaning of the present participle "combining" seems awkward and illogical.
- an infinitive such as "to make" often implies intention, as in "in order to made." Since these things were not intentionally done to make her a valuable historian, we should use a different construction.

(B) a vantage point, when combined with her talent for writing, that made
- the use of "when" suggests "only when," as Pratishtha pointed out.

(C) a vantage point that combined with her talent for writing, and it made
- the structure implies that the vantage point did 2 things: 1) combined with writing, and separately 2) made her a valuable historian. This is illogical. It is because she had the vantage point and the talent that she became a valuable historian. This structural change removes the causality.

(D) and this vantage point, which combined with her talent for writing to make
Fragment. If we begin ", and this vantage point [modifier]..." then "vantage point" is a subject of a new independent clause, and needs a verb. Everything after "which" is a modifying dependent clause, so this is a sentence fragment.

(E) and this vantage point, combined with her talent for writing, made
Correct! Full independent clause and logical modifiers.

Also... please edit to include your source!