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

Only the right answer will match the conclusion you highlight.

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

these elements are clearly still being created in the universe today.

[BACKGROUND]. However, even if this occurred, [CONCLUSION]. [SUPPORT]. So, [INTERMEDIATE CONCLUSION], but [SUPPORT].

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

(A) …have probably decayed into other, nonradioactive elements.

The author didn’t actually say that, only that “most of them decay”.

(B) Radioactive elements are being created in the universe today.

This maps pretty perfectly to the main conclusion.

(C) If no new radioactive elements…

This matches a statement that the author gave support for. Buuut, the author only brings it up to support that “these elements are clearly still being created”. That means it’s an intermediate conclusion, but not the “main conclusion”.

(D) It is possible that…

This matches the info in the background at the beginning of the passage, but there’s no support given for this.

(E) …most of the universe’s radioactive elements decay…

This matches part of the explanation in the author’s support, but there’s no support given to back this up. It’s not a conclusion.

(B) is the correct answer.