Since the last decade, Chinese firms have accounted for 90% of the int

Let's run through the wrong answers one at a time. What we know about the underlined portion is that it needs to fit in on both sides. On the left, it needs to be able to modify or otherwise follow the part about bamboo flooring. On the right, it needs to connect to the modifier "as Western consumers go green."


B. whose appeal grows
It's actually okay to say "whose" here. Strangely, while "who" refers to a person, we can use "whose" for any noun, simply because "which's" isn't a word. So I can say "The frisbee, whose popularity declined over the last decade . . . " Since that usage could create some misunderstanding in the context of this sentence, we may be better off without it, but that wouldn't completely eliminate B by itself. The other reason to cut B is the use of the simple present: "grows." Although this is not grammatically incorrect (it's parallel with "go green"), it doesn't convey the sense that the demand has been growing for some time. I'm not sure an official GMAT question would rely on these criteria for elimination, but A is definitely a better and clearer answer (as we'll discuss).

C. which has a growing appeal
D. a product with growing appeal

These are both fine on the left side. Either one can modify the preceding noun, "bamboo flooring," and since there's not much ambiguity here, there's no particular reason to pick D over C. The problem is on the right. "As Western consumers go green" is an adverbial modifier, and we need an action or clause for it to modify. This means we need to know WHAT IS HAPPENING as Western consumers go green.

D only provides a noun phrase: "a product with growing appeal." "Product" and "appeal" are nouns. "Growing" serves as an adjective modifying "appeal." There is no action here at all.
C might give us slight hope, since it at least contains a verb--"has." However, this doesn't help us to show the change over time that we need to convey. What has happened as consumers go green is not that this flooring HAS an appeal, growing or otherwise, but that the appeal has grown along with the progress of going green.

E. the appeal of the product has grown

This is a good old-fashioned comma splice, a.k.a. a run-on sentence. "The appeal of the product has grown" is an independent clause, so we can't just tack it on with a comma.

This leaves us with A. the appeal of which has grown

Not everyone may find this kind of noun modifier comfortable or familiar, but it's actually fairly common. We can say things like "I tried a new kind of coffee, the quality of which was unparalleled" or "He signed a contract, the terms of which he did not fully understand." It's basically a way to modify a noun we haven't yet used, but which is connected to a previous noun. I want to mention the appeal of the flooring, the quality of the coffee, or the terms of the contract I just mentioned. So this covers us up to "the appeal of which." We can read the sentence as if we've just said "The flooring has a great appeal, which . . . "

That gets us to the right-hand side. The use of present perfect shows us that the appeal has been in the process of growing. When? While the Western consumers have gone green. So why, you might ask, isn't that last part also in present perfect? That's the form I used just now in talking about the process: "Western consumers HAVE GONE green." Well, that's a bit tricky, but it has to do with completion. We can say that the appeal of something HAS GROWN without implying any sort of completion. The appeal can have grown and can continue to grow. But if I say that consumers "have gone green," it sounds like something that is done. "As consumers go green" more clearly indicates an ongoing process. Similarly, we might say "Computer skills have become more essential than ever as the world goes online." We can read general form as "X increases at a time when Y is in the process of happening."

I hope this helps! Let me know if I can clarify further.