According to damon, the first stage of building friendship for middle-school age children is

According to damon, the first stage of building friendship for middle-school age children is .  basing friendship in mutual trust and assistance.

Explanation:

All right to understand we need to recall Damon's classification of friendship development. Well, Damon considers children to live 3 different levels in their childhood period of friendship building. The first one is called level one and is based on "handy playtime", the second is called level two and is based on mutual "trust and assistance", and the last one is called level three and is based on intimacy and loyalty. So, that makes mutual trust and assistance the correct answer.