First, note the two choices for the first underlined word: "of" and "to." The correct idiom is "agree to" in cases like this. The treaty is what is to be agreed to. That eliminates (D) and (E).

Choice (B) may sound appealing if you only read the underlined portion, but when combined with the end of the sentence, there is the senseless construction "before the final treaty in March 1913 before the end of 1710." (C) changes the verb tenses in a way not wholly inappropriate (past tense for 1711 and 1713, past perfect for 1710), but both instances of "what was" are superfluous. (A) is structured in a way that all the included words are necessary, and it is correct.