Answer:
The answer is indeed the third sentence: (3) Some critics say renewable power has not improved pollution levels.
Explanation:
The author is clearly defending the use of renewable energy sources such as the sun, the wind, and the tides. According to him/her, those energy sources will reduce environmental impacts that we can now observe due to the use of fossil fuels. He/she mentions the opinion of critics, who defend the opposite point of view. According to such critics, pollution levels have not improved with the use of clean energy. That opposing view is mentioned in the third sentence.
(1) Renewable power is the best hope for meeting our energy needs while reducing environmental impacts. (2) Solar, wind, and tidal power are now possible. (3) Some critics say renewable power has not improved pollution levels. (4) But as the use of renewable resources spreads, benefits will be seen.
