Here are the suitable words to fill in the blanks:

1. A. coaches
2. B. atmosphere
3. A. group
4. C. do
5. A. knowledge
6. C. models

So the complete sentences would read as follows:

"Studies have proven that school club activities are important to both the physical and mental development of children. In sports clubs kids play sports regularly, and under the guidance of their **coaches** they improve their physical health. Moreover, children mix with friends from different classes in a new **atmosphere** where they can learn, share, and appreciate each other. Also, when they work with new friends in a **group**, they will become more confident and independent. Furthermore, when children work together in a **group** to deal with a problem or to do the task, they, therefore, can learn research and leadership **knowledge**. Many kids also find good role **models** in clubs, such as coaches or group leaders."