Einsteins great revelation was that a beam of light would not appear

OFFICIAL EXPLANATION






Mea culpa.
Large swear word.
My apologies to everyone.
Options A and B are both correct.
I forgot to add an error to option A. My mistake.

(I did not write this question. Its author identified option B as the OA. I think that she or he is wrong.)

The sentence compares the way a beam of light appears to a person on a moving train to the way a beam of light appears to a person watching from the tracks.

Take out the common ground (the way a beam of light appears to a person), and the more specific comparison is between two adjective phrases: on a moving train and watching from the tracks.

In options A and B, the comparison (contrast) is clear.
In terms of style, in a formal piece, such as one for Atlantic Monthly, I would retain the words "it would" in option B for the sake of clarity.
Another writer or editor might use the construction in option A to save space—to create a more concise sentence.

GMAC? Their writers would not forget to insert an error into option A.

Option C is incorrect because a beam of light does not appear FOR a person.

Options D and E are incorrect because they are not parallel.
→ In option D, "would not appear the same to" is not parallel to "a person would see it."
→ In option E, "would not appear the same to" is not parallel to "if a person would see it."

I will give everyone kudos, lock this thread, and ... not edit questions late at night. (Okay, mostly not.)

ElninoEffect and kaptainklutz , again, welcome.
thakurarun85 , I think I understand your question and answered it above. (I would retain "it would" for the sake of "extra" clarity and to preserve what I think is better flow.)

On the GMAT, you will not be faced with this kind of choice.
Furthermore, in cases that appear to be two close calls, one of them will almost always contain an error based on a different issue.

For being brave, everyone gets kudos.