With its plan to create a wildlife sanctuary out of previously unused

The book's explanation's explanation is often inadequate. I can only do so much here, but here's my 2 cents.

Many students have difficulty identifying which is the subject: "one" "a number of" or "industrialized nations"
We know "one" is singular. We know "a number of" is plural (NOT singular as someone mentioned). And of course the word following the phrase "a number of"--which in this case is "industrialized nations" will be plural.

so what do we use? singular or plural?

Well, the keyword here is "that"..By adding the word "that"..you are restricting the scope of industrialized nations you are referring to. You are not referring to ALL industrialized nations, but rather the ones THAT blah blah blah. Therefore, because of the word "that" you must use the plural form.

If the word "that" was not there, then you would use the singular form "is accepting"

But since we do have the word "that" and you are restricting the scope of industrialized nations that you are referring to, you must treat "a number of industrialized nations" as the subject, and use "are accepting"

That leaves us with C, D, and E. Of the three, only "their responsibility" makes sense in the context of "are accepting their responsibility"---you can eliminate C and D. The answer is E.

Again, I can only explain so much in text. This question falls into framework #7 of the GMAT Pill Study Method.