Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of punctuation between titles and proper nouns. No punctuation is needed to set off the coordinated noun phrase "Chile and Luxembourg" from the title that describes the two countries, "fellow OECD nations." Because the proper nouns "Chile and Luxembourg" are essential information identifying the "fellow OECD nations," no punctuation is necessary.
Why the other choices are wrong
Choice A
Choice A is incorrect because no punctuation is needed between the country names ("Chile and Luxembourg") and the title ("fellow OECD nations") describing them. Further, setting the countries’ names off with commas suggests that they could be removed without affecting the coherence of the sentence, which isn’t the case.
Choice B
Choice B is incorrect because no punctuation is needed between the country names ("Chile and Luxembourg") and the title ("fellow OECD nations") describing them.
Choice C
Choice C is incorrect because no punctuation is needed between the country names ("Chile and Luxembourg") and the title ("fellow OECD nations") describing them. Placing a colon after "nations" would confusingly suggest that these were the costs of the basket of goods for "fellow OECD nations" in general, not for Chile and Luxembourg specifically.