The difficulty with the proposed high-speed train line is that a used plane can be bought for one-third the price of the train line, and the plane, which is just as fast, can fly anywhere. The train would be a fixed linear system, and we live in a world that is spreading out in all directions and in which consumers choose the free-wheel systems (cars, buses, aircraft), which do not have fixed routes. Thus a sufficient market for the train will not exist.

Which of the following, if true, most severely weakens the argument presented above?

(A) Cars, buses, and planes require the efforts of drivers and pilots to guide them, whereas the train will be guided mechanically.
(B) Cars and buses are not nearly as fast as the high-speed train will be.
(C) Planes are not a free-wheel system because they can fly only between airports, which are less convenient for consumers than the high-speed train's stations would be.
(D) The high-speed train line cannot use currently underutilized train stations in large cities.
(E) For long trips, most people prefer to fly rather than to take ground-level transportation.

I agree with the OA.
However, it is tricky to find the conclusion (in blue) because the sentence in red seems to be the conclusion if you don't pay enough attention (It is at the end of the argument and used the word "thus").
In this sense, I would like to know whether there is a method or approach to find the conclusion in this type of tricky questions in a faster way. CR Powerscore Bible suggests to organize the ideas to find what statement is consequence of another statement; however, it takes time. Thanks!
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is true that the conclusion not always is founded at the end of the stimulus (98% of time YES).

However, i disagree with you; for me the red phrase is the conclusion, and even not, the key point of the entire situation is that train are NOT convenient.

We have to find something that says train IS convenient. C does.

During this (huge, monster, crazy , amazing, unbelievable) exam one of the central point is : understand what's going on on the problem. As Ron says: be flexible

Without consider or not the conclusion.

First sentence: The difficulty with the proposed high-speed train line is that a used plane can be bought for one-third the price of the train line, and the plane, which is just as fast, can fly anywhere. -------> what 's that mean: train as more expensive than plane.

Second sentence: he train would be a fixed linear system, and we live in a world that is spreading out in all directions and in which consumers choose the free-wheel systems (cars, buses, aircraft), which do not have fixed routes ---------> we live in a world with a lot of movement where wheels are a problem rather than plane.

Thierd sentence: Thus a sufficient market for the train will not exist ----------> Train has no (or will not) market sufficient enough to develope.

I do not see the process above takes much time. At most 20 seconds. This collimate with what CR Bible says

That's it
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CASE 1

Premise - The difficulty with the proposed high-speed train line is that a used plane can be bought for one-third the price of the train line, and the plane, which is just as fast, can fly anywhere. The train would be a fixed linear system, and we live in a world that is spreading out in all directions and in which consumers choose the free-wheel systems (cars, buses, aircraft), which do not have fixed routes.
Conclusion - Thus a sufficient market for the train will not exist.

Anything which weakens the conclusion is the answer or anything which strengthen that "the sufficient market for train will exist"

(A) Cars, buses, and planes require the efforts of drivers and pilots to guide them, whereas the train will be guided mechanically. (It does prove that it will affect in anyway the market)
(B) Cars and buses are not nearly as fast as the high-speed train will be. (Even planes are fast, this sentence has ignored planes so not relevant)
(C) Planes are not a free-wheel system because they can fly only between airports, which are less convenient for consumers than the high-speed train's stations would be. (This weakens the conslusion by showing that there would be sufficient market - This is our answer)
(D) The high-speed train line cannot use currently underutilized train stations in large cities. (Not relevant)
(E) For long trips, most people prefer to fly rather than to take ground-level transportation. (Strengthen the conclusion)

CASE 2

Premise - The train would be a fixed linear system, and we live in a world that is spreading out in all directions and in which consumers choose the free-wheel systems (cars, buses, aircraft), which do not have fixed routes.
Conclusion - The difficulty with the proposed high-speed train line is that a used plane can be bought for one-third the price of the train line, and the plane, which is just as fast, can fly anywhere.

Anything which weakens the conclusion is the answer or anything which strengthen that "the plane cannot fly anywhere" is the answer

Even in this case only option C makes sense.

But rightly said by metallicafan, prsence of "thus" pretty much assure us of the conclusion.
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First Attempt 710 - first-attempt-141273.html

First of all, we need to know the intention of author (or main point of the passage/argument). The author is intended to convince the readers that, in future, planes will be better than train.

Logical Structure:
Premise 1: Compared to high speed trains, planes don't cost so much as to deter their purchase.
Premise 2: Planes and High Speed Trains travel with comparable speeds.
Premise 3: Unlike Planes, trains have fixed routes.
Analogy 1: Bus and cars, which don't have fixed routes, are preferred by customers.

Conclusion : Hence, Trains will not be preferred to planes. (Market does not exist for trains)

Because this is a weaken question, we will be thrown some new info which will disrupt the logical flow or question the assumptions/analogies or expose some flaws in the causal relationship. So, let us check the new info provided in options.

(A) Cars, buses, and planes require the efforts of drivers and pilots to guide them, whereas the train will be guided mechanically.
This info, at most, builds a new premise that free-guidance might be a factor in customer's decision of transportation. But no info has been given whether 'guidance' will help or deter train transportation.

(B) Cars and buses are not nearly as fast as the high-speed train will be.
The real intention is comparing between plane and high speed trains. (Cars and buses info is just to establish analogy between free-wheel systems)

(C) Planes are not a free-wheel system because they can fly only between airports, which are less convenient for consumers than the high-speed train's stations would be.
This info tells us that, for consumers, train stations are convenient than airports. So this inserts a new info speaks against the intention of author.

(D) The high-speed train line cannot use currently underutilized train stations in large cities.
If the under-utilized stations cannot be used by high speed trains, train should not be preferred.

(E) For long trips, most people prefer to fly rather than to take ground-level transportation.
It says that personal choices make plane preferred transportation.

Only Option C speaks something relevant enough against the conclusion of the argument. So C weakens the argument.