The main issue being tested here is parallelism. Whenever there are lists or comparisons, we want nouns to be compared to nouns, verbs to be compared to verbs etc.
In your example, the items listed in parallel are: reversals, the onset, splitting, and great volcanic eruptions.
Reversals, the onset, and volcanic eruptions are all nouns, only splitting is in verb form. We need to find an answer that changes this word to the noun form.
Answers A and C still have split in verb form. Incorrect.
answers D and E correctly use a noun, but the change in word order changes the original intent of the sentence. The focus of this list is on the actual splitting apart of continents, not the continent themselves. Eliminate
Only answer B correctly changes splitting to a noun form by adding the article "the."
Focused GMAT Prep