When a teacher and her students develop a feeling of harmony, well-being and security towards each other, this is a sign of?

- Rapport existing.

Explanation:

As per the question, when the teacher and his/her students succeed in developing a feeling of harmony, welfare, and a sense of security or reliability towards each other; the situation signifies the 'existence of rapport' between them. Rapport promotes a relationship of mutual faith and dignity towards each other (involved in the relationship). In the given context, the rapport encourages a close bond (between the students and the teachers) filled with harmony, understanding, and empathy towards each other that makes them able to communicate, believe, and interact more effectively.