Explanation

The blank calls for words that characterize the types of environmental problems worth spending large sums on. Among the choices are two sets of near synonyms: “ephemeral/transitory” and “pressing/critical.” Of these two pairings, only the latter makes sense in the context of the rest of the sentence—it would be unlikely for anyone to advocate spending large sums on problems that will be quickly gone without any intervention, as “ephemeral” and “transitory” imply, nor would unsanitary water likely be characterized as a merely temporary dilemma. Of the other two options, neither “futile” nor “controversial” has a synonym among the answer choices, so they can be ruled out on that account; in addition, neither would be used to justify the expenditure of large sums of money. Thus the correct answer is pressing (Choice C) and critical (Choice F).

In 'unglamorous but pressing', how 'but' is used?

If we consider spending money to repair the damages we have done to the environment, it is also better to spend a certain amount of that money to fix problems that apparently are unglamorous, which means they do not have the same media coverage or a lot of resonance in the public opinion, BUT (at the same time, nonetheless) are ESSENTIAL or critical.

Hope now is more clear.

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