How will the right answer fit in terms of support and conclusion?

Only the right answer will be support that must be true if the conclusion is true, which is a more precise way of saying only the right answer will support the conclusion without bringing in anything new.

Highlight the main conclusion in the passage, if there is one:

we should be able to determine how a star’s relative actual brightness correlates with other characteristics by studying stars in the same distant galaxy.

[SUPPORT]. Thus, [INTERMEDIATE CONCLUSION]. Therefore, [MAIN CONCLUSION].

Even with this really wordy conclusion, you’d like to catch that the author changes the subject. There’s nothing in the rest of the passage about “other characteristics”, so there’s no support for the author saying that’s how to learn about them. The right answer will absolutely call out that new wording in the conclusion.

Map the wording of the answers to the wording of the passage:

(A) If two stars are in two different galaxies…

…then those stars are in a different argument. The author only talks about stars “in the same distant galaxy.”

(B) …the elements each is burning…

…are never mentioned in the passage. It mentions “burning”, but don’t assume the “elements” have to be one of the “other characteristics”. This answer is bringing in something new, so the conclusion could work even if this isn’t true.

(C) The stars in our own galaxy…

…are never mentioned in the passage. The argument is only about stars in a “distant galaxy.”

(D) …that have characteristics, other than brightness…

Yup, if “we should be able to determine” stuff about “other characteristics”, then it must be true that those characteristics are “discernible”. This is the only answer that maps to the “other characteristics” in the conclusion without getting too specific like (B).

(E) If there are significant differences in how far away…

Stop. The author says that’s not true, so we don’t care what would happen if it was.

(D) is the correct answer.