His definition of public opinion, the author states that the opinions of private individuals become public opinion when they

I believe the answer is: are openly expressed

Private opinion could only exist in one person's mind,. While it might be provide the most safety and unlikely to cause conflict, it would never be able to change how people think or behave. the second that individual express their opinion to other people, it create a high chance of causing a conflict. But, the chance of other people hearing it and change their opinion goes up along with it.