There are several issues at play in this argument, chiefly MEANING, PARALLELISM, and MODIFIERS.

When reading an SC sentence, you first goal is to understand the meaning. In this case:
cloud of particles --> triggered a storm --> brightened Northern lights and knocked out satellite

In any given sentence, though, it's possible to change the order of various pieces and still maintain the same meaning. So, instead of having "cloud" as the subject, it's certainly possible, as you suggested, to have "storm" as the subject:
a storm (triggered by the cloud) --> brightened Northern lights and knocked out a satellite

However, we also need to consider any meaning shift that would result from the change in structure. Because our initial modifier is "According to scientists who monitored its path..." Shifting the subject would shift what scientists are monitoring. Since scientists could monitor the path of a cloud or the path of a storm, it's better to look to other issues.

So, you don't always have to make a decision on what the correct subject is - there may be several potentially correct versions. Instead of looking for your preferred version of the sentence, look for the differences between each answer choice, and ask yourself a) is there a meaning difference, and b) is there a difference in grammar?

(A) an expanding cloud of energized particles ejected from the Sun recently triggered a large storm in the magnetic field that surrounds Earth, which brightened the Northern Lights and also possibly knocking

BRIGHTENED and KNOCKING should be parallel.


(B) an expanding cloud of energized particles ejected from the Sun was what recently triggered a large storm in the magnetic field that surrounds Earth, and it brightened the Northern Lights and also possibly knocked

The continuity of meaning here is not correct. We need to suggest a cause and effect between triggering the storm and brightening the Northern lights. THis treats them as separate actions. It "was was triggered" is not succinct.


(C) an expanding cloud of energized particles ejected from the Sun recently triggered a large storm in the magnetic field that surrounds Earth, brightening the Northern Lights and possibly knocking

Correct. BRIGHTENING and KNOCKING are parallel, and the cause-effect structure is efficiently conveyed.


(D) a large storm in the magnetic field that surrounds Earth, recently triggered by an expanding cloud of energized particles, brightened the Northern Lights and it possibly knocked

It may be ok to switch the subject to "storm", although the shift in emphasis is a slight shift in meaning: now the scientists are monitoring the storm rather than the cloud. More importantly, "the storm... brightened and it knocked" is not a parallel construction. Also, by placing "recently triggered" so far from "storm," there is some modifier ambiguity: it could be Earth that's triggered, or the magnetic field that surrounds the Earth.


(E) a large storm in the magnetic field surrounding Earth was recently triggered by an expanding cloud of energized particles, brightening the Northern Lights and it possibly knocked

Same meaning shift as in D. BRIGHTENING and KNOCKED are not parallel.