I'm not exactly sure what it's asking but I think that A and D are equal in weight which makes it able to push D and for D to be able to move at the same speed. But I think that C is heavier than A and D because when A hits it slows down.
Asexual reproduction involves a simple exact copying of the genetic material. An exact copying is FAR less likely to result in mutations, which cause variation. The other three answers lead to mutations which cause variation.