According to scientists who monitored its path, an expanding cloud

Hi clover1987,

By modifier, do you mean the ejected/triggered split or are you referring to recently? We can start with triggered, which is the main verb here. We cannot assume that it is a (passive) participle modifier.

... an expanding cloud of energized particles ejected from the Sun recently triggered a large storm in the magnetic field that surrounds Earth...

1. If triggered were a modifier, we would not have been able to add "a large storm..." (an object, something that tells us what was triggered) after it.

A partial meltdown triggered by a faulty reactor design... ← Triggered is a participle here. Because it is passive, there is no "what it triggered" after it. Also, this is not a complete sentence.
A faulty reactor design triggered a partial meltdown. ← Triggered is a complete (active) verb here. Given this meaning of triggered (triggered is transitive here), we must add a "what it triggered" after it. This is a complete sentence.

2. Similarly, because it is unlikely that the cloud (or particles) ejected like a pilot, we can assume that ejected is passive in this sentence.

an expanding cloud of energized particles ejected from the Sun ~ an expanding cloud of energized particles that was ejected from the Sun (was for cloud, were if the reference is to particles)

Because (1) effectively means that triggered must be a verb, and (2) tells us that ejected cannot be a verb, we can be absolutely sure that triggered combines with cloud to create a subject-verb combination.

All of this is to say that we should not ask "was it triggered", because the cloud was not triggered. Instead, the cloud triggered something else (a large storm).

Now, if you were worried about the recently, my guess is that recently is meant to describe triggered, because it is not very close to ejected. The thing to keep in mind here though, is that it (recently) introduces a little bit of ambiguity in the sentence. That is all. This is not a major issue, and option C is still much better than the other options.