The author here is taking issue with people "arguing that there was no trade between Europe and East Asia in the early Middle Ages because there are no written records of such trade." This is the conclusion, i.e. that you cannot argue this.
The support provided is an example of the yeti and the fact that a verifiable sighting of the yeti would prove that the creature does exist, but the absence of sightings cannot prove that it does not.
We are being asked to identify the main point of this argument.
(C) states, "That trade between Europe and East Asia did not exist in the early Middle Ages cannot be established simply by the absence of a certain sort of evidence that this trade existed."
So we cannot establish that trade between Europe and East Asia did not exist in the early Middle Ages simply by an absence of evidence that this trade existed.
This is the point of the argument so (C) is the correct answer.
(A) on the other hand while true is not the main point of this argument. Remember there are two criteria for the correct answer on a Main Point question: (1) Must Be True and (2) Main Point.
The author's point is not that this evidence is not scientifically confirmed. It is that you cannot argue that trade did not exist because this evidence has not been scientifically confirmed.
Hope this helps! Please let us know if you have any other questions.
