Answer: B

This is a strengthen question. It is similar, however, to an explanation, as the argument contains an apparent paradox. Developed countries will cut emissions by 50%, developing countries by 23%, but the 23% cuts will be more demanding on a per-capita basis. The evidence given is that developing countries will need more resources to continue growing. Presumably, developed nations will not need their resource consumption to grow at the same rate. Consider each choice:

(A) This choice describes one target of emissions cuts, but in developed nations. It doesn't explain why developing nations will be hit harder.
(B) This is correct. If emissions in developing countries start at so low a point, they would need to grow very rapidly to get anywhere close to the level of developed nations. As the resource demands in those countries grow, any cut at all--let alone 23%--in emissions would be a great burden.
(C) This choice distinguishes some developing nations from other developing nations. The contrast we're concerned with is developing vs. developed nations.
(D) This may be true, but it doesn't tell us why emissions cuts would be so much more harmful. If anything, it may mean the cuts would be less harmful, as they would disproportionately touch the ruling class.
(E) This is something that is partly assumed by the argument, but without additional information (in the form of details regarding emissions levels, as in (B)) it doesn't strengthen the argument.