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

Only the right answer will be support for the conclusion in the prompt that there’s “an alternative explanation”.

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

the organic matter stored by the ants provides those orchids with additional nutrients.

[BACKGROUND]. [SUPPORT]. Since [SUPPORT], an explanation for the superior health of the orchids that house ants is that [CONCLUSION].

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

(A) Microscopic organisms that are present…

It’s fair for an “alternative explanation” to bring in something new, but the right answer has to fill in how this explains “the superior health of the orchids that house ants”. This answer leaves us guessing about that.

(B) …the same as those present in the soil in which the orchids are found.

This is making a new comparison between different soil, which doesn’t map to the comparison between different orchids in the passage. You’d have to fill in your own reasoning to connect this to “an explanation for the superior health of the orchids”, so it’s out.

(C) …prey on an insect species whose members eat the orchids’ roots.

So the ants basically protect their orchids from having their roots eaten. That clearly explains why those orchids would have “superior health” without us needing to make our own assumption.

(D) …do not play a role in pollinating the flowers of the plant.

Cool story, but no way “pollinating” is directly related to explaining orchids’ “superior health” without any more info.

(E) Most plant species whose…

Stop. The passage only gives support and makes an argument about “orchids”, so info about other “plant species” isn’t particularly relevant.

(C) is the correct answer.