Explanation

Creating a filter: In this question, when we read the prompt, we pause about the relationship between the word "if" and the word "were." As usual, we're checking not just for grammar, but to identify what the intended meaning of the sentence is and whether it's expressed. The word "if" identifies a condition. "Were" is a verb in the subjunctive mood, which in this case, is expressing a condition contrary to fact. Those two things could go together. We could say, "if we were king, we would do X, Y, and Z." But this is a statement about the future. The words "will strengthen" and "someday" make that clear. And in the future, the condition doesn't go along with the contrary to fact. It's a fact: if these countries place pressure, they will strengthen their charter.

Applying the filter: This leads us to answer choice (E). Choice (B) uses the passive tense without need. Choice (C) uses "would" inside the condition, which doesn't make sense here. (D) has the same problem as (A). The answer is (E). Notice that the word "but" isn't needed. The phrase "only if" is accurate and complete in identifying the necessary condition for the strengthening of the charter. The correct answer is (E).