Hi Shira!
Since this is a Weaken question, let's start by discussing what we're trying to weaken before moving into the answer choices.
This question specifically refers to the argument presented in lines 57-62, which we can interpret to be arguing that because termination and modification clauses are written into contracts, we can conclude termination/modification is not an inherent power. Phrased another way, the author is saying that if these powers were inherent, we would not need to write them into contracts.
In order to weaken this argument, we want to look for something that shows what you mentioned, that assumed/inherent things are at least sometimes written into contracts. This would weaken the argument by making it less likely that the inclusion of termination/modification clauses is indicative of a lack of inherent state power.
Answer choice (A) perfectly counters the argument for this exact reason! If specific provisions often contain restatements of what people already agree to be inherent powers, how could it be the case that such restatements mean those inherent powers do not exist? That wouldn't make sense -- both parties already agree that these inherent powers do exist!
Answer choice (E), on the other hand, doesn't really do much to weaken the argument. Does it matter if termination/modification agreements are interpreted differently by national courts and international tribunals? Does differing interpretations make it less likely that the presence of termination/modification clauses indicates a lack of inherent state power? No -- differing interpretations seems irrelevant! So, we can eliminate (E) in favor of (A).
I hope this helps!
Kate