@dianne_huxley
I suspect I'm reading your question incorrectly, so let me answer both ways I can think of.
To reply to an individual student's discussion post, follow this lesson from the Canvas Instructor Guide: How do I reply to a discussion as an instructor? This will make the reply available for everyone to read.
If your question is about how to reply so that only the individual student can see it, then the comment cannot be left in the discussion itself. You will need to leave a submission (assignment) comment or send a message through the inbox. The submission comment is tied to the assignment, but students sometimes have trouble finding it (this is supposed to be addressed in upcoming changes to Canvas).
The ways to send comments individually to students are many. I recommend checking out the Canvas Instructor Guide for even more choice than I'm providing here.
