Hi, Mok,
This question asks you to identify an assumption necessary for the argument to be valid. In other words, you must identify some unstated belief implicit in the argument that is required for the conclusion to make any sense. The key word in the question stem is "required." Which among the answer choices is "required" for the conclusion to be valid?
When you start by working the stimulus, identify the conclusion:
The Dörpfeld excavation did not uncover the remains of ancient Troy.
Next find the facts supporting this conclusion:
The city uncovered was too small to have withstood a ten-year siege.
In the ancient epic The Iliad, Troy is subjected to a ten-year siege.
Now, ask yourself why you might disagree with the conclusion. How is it possible that the city that was found could have been ancient Troy even though the city was clearly too small to have been subjected to the siege depicted in the poem?
Your job is to illustrate an alternate scenario consistent with the facts. In this case, you might come up with the idea, "What if the story is not completely accurate?"
What you do here is identify the gap in the reasoning, a problem that illustrates that even though you have some support for the author's conclusion, it is still possible to arrive at alternate outcomes.
Now consider the question stem:
What is an assumption (missing, implicit premise) required by the argument?
What must the author believe at a minimum for his conclusion to be valid? Go back to your observation: What if the story is not true?
Now connect your observation to the question stem task to come up with a prephrase:
At a minimum the author must believe the events related in
The Iliad represent a somewhat accurate description of actual history.
With this prephrase you arrive at answer choice E. Imagine that E were not true,
The Iliad does not accurately represent the duration of the Trojan War. In this case, the conclusion of the argument makes no sense. This truth of this information is clearly required for the argument to be valid.
Now consider your answer, C: Is it really necessary that Dörpfeld's team found no evidence that a siege had occurred in the city they found? No. Even if this team had found such evidence, the city in question still might not have been ancient Troy. It could have been some other besieged city.
I hope this explanation helps.