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

The prompt alone doesn’t actually tell us, since either support or a conclusion could fill in the blank. “Apparently” is a good way to tell quickly that in this case, only the right answer will be a conclusion supported by this long ass passage.

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

There is no conclusion in the passage, we’re being asked to fill it in.

[BACKGROUND]. [BACKGROUND]. [BACKGROUND]. [SUPPORT]. Apparently, [CONCLUSION].

You don’t really need anything in the first three sentences. The idea is just that if traveling mu mesons decayed “as fast as they do in the laboratory”, we would get a different result than the one we actually get. So, I guess those mu mesons do NOT decay as fast as they do in the lab.

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

(D) decay more slowly…

This is the only answer that maps to the only piece of support given, about how fast stuff decays. Savvy test-takers will spend very little time thinking about any of the others.

(A) take longer to reach Earth than was supposed

How long were we supposing they take? There’s no info about that in the passage, only the speed they go.

(B) are quite difficult to detect…

The support says we detect 100 times more than the whole decay speed thing says we should. Definitely no support for detection being “difficult”.

(C) are much less numerous than previously thought

How numerous did we previously think they were? There’s nothing about that in the passage.

(E) are probably not generated by cosmic rays

“Nuh uhhh!” answers like this are always wrong. The passage explicitly told us they’re “generated by cosmic rays”. Don’t fight the passage.

(D) is the correct answer.