Explanation

Creating a filter: When we read the prompt, we find two flags in the underlined portion, the pronoun "those," and also a comparison, which contains the pronoun. The vague pronoun is muddling the comparison. Both logically and grammatically, what are we exactly comparing -- barrels, years, or reserves? Or stockpiles? It can only be one. We have a filter, so we look for an answer choice to dispel this concern.

Applying the filter: Choices (C) and (D) are both simple and thus tempting. But they compare "1,000 barrels" with a year, 2003. We can't compare barrels with a year. Look for more objective problems, we see that choice (E) says "fewer than that." What is "that" compared to? It can't be "barrels," because that's plural, and it can't be "stockpile," because (E) mentions a stockpile later. So (E) has a bad reference and is out. We want to get rid of (A) now. The imaginary author is trying to say that the stockpile becomes 20 percent smaller. The stockpile became 1,000 barrels, according to the sentence. And the words "stockpile" and "reserves" are synonymous; they can be compared. That means (B) is okay. And, finally, we can see a defect in choice (A): the word "those," being plural, would have to refer to "barrels," but then we would have to use "fewer," not "less," since barrels are countable items. The comparison word "fewer" must be used with countable items and the word "less" with non-countable quantities. The correct answer is (B).