Often times, simply underlining a passage is enough to draw a students attention. It also leaves space for them to critically think through their writing choices and assess what the problem might be. However is the writing is far from meeting expectations, this might not be enough to help guide the student in the right direction.
Phrasing feedback as a question helps motivate student thinking.
Sometimes specific detail can be helpful, other times it can reduce students to thinking in a "checklist" fashion about revisions.
Try to think about how your feedback may help students refocus on the audience and purpose of the writing.