Explanation

Creating a filter: In this question, we don't have to know Einstein's physics to answer correctly, but we do need a sense of the sentence's intended meaning. People go astray on the GMAT all the time by trying to apply pure grammar and style rules without considering the intended meaning of the sentence. We are primarily comparing how light looks to a moving person and how it looks to a stationary person. Grammatically, we can extract the modifying phrase "moving alongside the beam at the same speed" without altering the surrounding grammar of the sentence. That leaves "would look the same to an observer... as an observer who was stationary." That doesn't sound terrible, but it's not a parallel comparison.

Applying the filter: choice (B) gives a parallel comparison.

Further notes: choice (A) is also ambiguous. It could also be taken to mean "would look the same to an observer.... as an observer who was stationary would look." Totally different meaning. (C), (D), and (E) all variously include unnecessary punctuation, repetition of the pronoun, and the word "just."

The correct answer is (B).