<p>The correct answer is (B) that.</p><p>Opinion polls last month indicate <strong>that</strong> only 40 percent of residents were in favor of extending the lifespan of existing nuclear power plants.</p>
<p>In this sentence, the word "that" is the correct choice. It is used to introduce a clause that provides more information about the opinion polls. The clause "only 40 percent of residents were in favor of extending the lifespan of existing nuclear power plants" is the direct object of the verb "indicate." The word "that" is commonly used in this context to introduce a noun clause that functions as the direct object of the verb. The other options do not fit grammatically or contextually in this sentence.</p>