GMAT Sentence Correction (SC) | Re: With its plan to create a wildlife sanctuary out of previously unused

Look at the difference between the two examples given below.
Japan, one of a number of top industrialized nations is rich – singular verb because one is the subject.
Japan, one of a number of top industrialized nations that are rich – that modifies the nations and as a relative pronoun stands for the plural nations as a plural subject, entailing a plural verb- are rich-.

---