Why should someone explain? Are you giving us a quiz or something? Give us your opinions on what you think of the question and we will then have a lively discussion. Or do you want others to do your homework?
why pre colonial Africa societies were classless,dynamic and unchanged?
This question is too Ambiguous. How possible can one thing be "Dynamic" and yet "Unchanged"?
Talking about class, we have seen that before the intrusion of colonial Powers, African societies where experiencing the class divisions just like other societies; as there were Upper class (Kings like Mkwawa et al, Business men who were exchanging goods with Arabs etc) and then class of regular citizens (call it Middle or low class, as you wish!)
I hope my answer may contain at least some of what you are looking for..