I've been practising a couple of CR questions in my learning process and this is by far the most difficult CR question I have come across. I will have to say I got this one wrong initially but after much thought I feel my retrospective position on the question is the following:

Based on my reading of the main argument the Conclusion is: ""increases in governmental spending on research into energy technology will be necessary if we wish to reduce CO2 without compromising on growth".

This position is derived from one main premise:
1. The current technologies will not be able to reduce emissions without hurting economic growth

Besides this no other key information is provided to frame the conclusion the argument provides and thus clearly there is an assumption required here to fill the vary apparent gap in this argument.

The only piece of information not available with some context or premise is the inclusion of the term "government spending" and why only it is the only necessary condition for technology research to take place. Thus, the assumption has to be that government funding is better at doing this sort of research than any other funding i.e non-governmental funding.

Thus, answer B -- very nasty question.

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A lot of people forget that in the GMAT when an argument is made, we deny any other possibility of ariving at that same argument via a different route.

Enter: Defender Assumptions

The Argument is that government spending on research into energy tech is Necessary to reduce carbon dioxide without stifling economic growth.

B is Correct because the argument is that government spending is necessary. Therefore it must be true that non-government spending is less likely than government spending into energy tech research is to reduce emissions since the argument only states that government spending is necessary.

If you negate B you will see that B weakens the argument:
Increased governmental spending on research into energy technology will not be more likely to reduce carbon dioxide emissions without stifling growth than will nongovernmental spending.

So why is government spending necessary?

Here is why each incorrect answer is incorrect:
Economist: Even with energy conservation efforts, current technologies cannot support both a reduction in carbon dioxide emissions and an expanding global economy. Attempts to restrain emissions without new technology will stifle economic growth. Therefore, increases in governmental spending on research into energy technology will be necessary if we wish to reduce carbon dioxide emissions without stifling economic growth.

A. Incorrect. The relationship in the argument is concerned with simultaneously reducing emissions while expanding the global economy. This answer choice confuses the relationship. All we know is that Research into tech is NECESSARY in order to simultaneously grow the economy and reduce emissions (sufficient).

C. Incorrect. Government research is required for an expanding economy that is also looking to reduce emissions at the same time without affecting growth. This statement is incomplete.

D. Incorrect. This doesn't need to be true for the argument that research into tech is necessary in order to simultaneously grow the economy and reduce emissions to be true. In fact this is a failed inference. We just know that new tech is required to support reductions in emissions without stiffling the economy.

E. Incorrect for similar reasons to D. We just know that current tech combined with conservation efforts aren't enough. The argument is made regarding new technology research.
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Here is my 2 cents.

Many people including me might think that "nongovernmental spending" is not the topic of this problem so it is irrelevant. We actually did encounter many other problems stating this kind of conclusion: X will increase Y. And there might be a wrong choice: Z is more likely to increase Y. In this case, this choice is definitely irrelevant.
However, let's go back to this OG problem: notice that we have "necessary" in the conclusion. Here, "necessary" means no other sources could be a substitute. Therefore, "nongovernmental spending" is actually relevant now.
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If B did not have the "than non-governmental (NG)" clause, then it would have been a sureshot right answer.

However, since there is a comparison with an item not mentioned in the premise, much like other questions, this becomes a case of irrelevance/out of scope.

Negating B does say that Governemnental (G) is not more likely than (NG), however, it still might be effective at the said purpose to some extent and will not "destroy" the argument.

IMO, if one says "X is necessary' that doesn`t mean uniqueness of x. There can be other options equal to/or superior to X, yielding the same or better results.