This is a tricky question.
IMO the point they are making is that, an object moving from far away appears to be moving slower than when the same object is closer. This is the premise.
Conclusion is:
The approximation is typically lower when its farther than when its closer.
This actually logically follows only if you assume that the speed of car was always the same across both points measured. If the speed were different then the approximation wont make any sense.
Hence
option C is the correct answer IMO.
Option E IMO looks tempting but that is something that is inferred from the passage and not something that needs to be an assumption. The stem is cleverly disguised in a way to think it might be an inference when I think its indirectly asking for an assumption.
You have a premise, conclusion, what is something if stated true would make the above statements make sense as well? Rephrase it and: What is the assumption needed for the above statements?
dp1234
Greater FoV = object appear slower
Car far away -> appear slower (than actual) and car nearer -> appear less slower (than actual)
Thus approximation of speed for closer car is better than that of far away car.
IMO E.