Official Explanation
Since the tremendous wealth on display represents only a tiny fraction of the collection, and since the collection has “over one million specimens,” it is a very large collection indeed. You might anticipate an answer like huge. “Exhaustive,” which means including everything, and “sweeping,” which means extensive, both fit. If a collection is “piecemeal,” it would have been acquired piece by piece over time. Likely as that might be, there’s no clue in the sentence that would suggest this. “Voluble” means talkative and does not fit, nor does “evergreen,” which means fresh or popular. Finally, “commanding” means authoritative or imposing, and there’s no reason to believe that the collection commands authority over other collections.
Answer: C,E