The right answer is "Students who read below grade level are twice as likely to drop out of high school than those who read at or above grade level. "
The explanation for this answer to be considered the correct alternative is that the question asks to show which of the alternatives presents a fact or statistic about the benefit of the reading, thus, this is the only alterative, among those that were exposed, that presents a benefit of reading. This benefit is the high probability of completing high school that students who have a habit of reading have.