A. Employers can use online activity AGAINST you.
Dangers of social media would have a negative connotation, which is unlike options C and D as they are rather positive/effective uses of social media. So C and D are incorrect. Option B doesn’t specify that the interest group is bad, so I’m inclined to say that’s not correct either. Option A seems like the best answer in this context. Something used “against you,” would cause you harm, pushback, difficulty, or opposition.