The correct option is B They think
Let’s examine the sentence: “When many people think about termites, [1] you think about having to call the exterminator.” The sentence talks about ‘people’ and so it would use the pronoun that can be used for ‘people’ only. ‘They’ is the pronoun used for people. Hence, ‘you’ would be replaced by ‘they.’ So, Option B is the correct answer.