The detail that supports an argument that technology brings some teens together because it improves their social lives is “[f]ifteen to 30 percent of teens . . . reported feeling less shy, more outgoing, more self-confident, more popular”.
The paragraph starts with the topic, first, it refers to teens who do not feel social networks change their social lives. Second, it refers to those who claim social media has a positive effect on their social lives and, finally, it refers to those teens who feel social media has a negative impact on their lives.
The sentence “[f]ifteen to 30 percent of teens . . . reported feeling less shy, more outgoing, more self-confident, more popular” belongs to the second aspect described. That is to say, it is supporting the claim about the positive impact of social media in teens' lives.
The other options are not correct because the first one is the topic sentence, and the last option is a negative effect of social networks on teens' lives.