My understanding:Subject and Verb: "Phenomenon" is singular, so "occurs" is correct. "Occur" would be used for plural subjects.
Modifiers:
"Explainable" modifies the whole sentence, indicating that only a few scientists can explain the phenomenon.
The "only" in "occurs" limits the reason for the mass suicide, implying it happens only after a majority of the animals migrate.
Correct Verb Agreement: In “after a majority of the animals is driven to migrate,” the subject “a majority” should take the plural verb “are driven” instead of “is.”
Between C and D: The structure in D is "The phenomenon is explainable" suggesting that the phenomenon is always explainable, which might be too strong.
Also, there should be an "and" between "students" and "occurs".
For option E: The phrase "it occurs" is less concise and adds unnecessary complexity. If the structure would have been "can be explained" and "occurs" then the sentence would be more clearer.