To respond to a comment, click "Reply".
To respond to activities, click the corresponding activity from the activity feed. A new window will open where you can leave an activity comment. Click on "Send" to send the comment.
You can respond to activities from the activity area on the left side of the screen (in the mobile version via the speech bubble icon at the top right of the screen), project activities, and task activities. The user who performed the activity is automatically @mentioned, so they are notified of your comment. The response will be displayed as a comment on the task.