Garibaldi captured Rome in 1870, in the final step in Italian Reunific

Magoosh Official Explanation

Split #1: verb tense. Garibaldi capture Rome, and then as a result, the Vatican took some action. The action that the Vatican took had to happen either more or less at the same time as Garibaldi's actions or even slightly later, so we definitely do not need the past perfect ("had ceased"). The present perfect ("has ceased") is awkward, because it would suggest that the "ceasing" is an action that continues up to the present day, but that makes no sense --- "to cease" is a one-time action. Therefore the only tense that makes sense is the simple past ("ceased"): this is what choices (D) & (E) have.

Split #2: parallelism. Should the final verb ("become"/ "becoming"/ "became") be in parallel to the "ceasing" or to the "holding"? What is the sense of the prompt? The prompt communicates that the Vatican did two actions. One action was ceasing to hold an army. The other action was becoming a purely religious institution. Those two verbs, "cease" and "become", must be in parallel. Choices (C) & (D) make the mistake of putting "become" in parallel with "holding".

For all these reasons, (E) is the only possible answer.