C). After sentence 6 to support the topic sentence.
Explanation:
The sentence 'researchers are ...at all times' should be added after sentence 6 as it may best function as support to the topic sentence. <u>The topic sentence discusses that the nation is largely dependent on fossil fuel for its energy usage and sentence 6 elaborates this point that only 1% of energy is generated through alternative sources</u>. Therefore, the given sentence would serve as evidence to support this claim by stating about the researchers' efforts to find alternatives to reduce this dependency. This substantiate the author's point before the reader and hence, <u>option C</u> is the correct answer.
