GMAT Sentence Correction (SC) | A number of good reasons and A wealth of good reasons.

A wealth of good reasons is to say that there are several good reasons, umpteen reasons, or many good reasons. It is essentially plural IMO. Therefore, there are at least three choices that use the plural verb or. The only issue is whether one would use, doubtlessly, undoubtedly or without a doubt. All the three adverbs mean almost the same thing except for shades of difference in their relative strength. I doubt, whether GMAT tests shades of difference in meaning, which is a vocabulary test.