A. It refers to the definition of full-contact sports, where the condition is that the sport must have athletes must "purposely collide with each other" to be able to fit into that definition and category of full-contact sport.
B, C, and D are not definition statements. B is an opinion, since it claims that full-contact sports are "bad", which may be subjective from person to person. C is also an opinion, because parents "should not" allow children, tapping on parents instinct to feel "responsible". D is a statement of fact/statistic.
Hope this helps! :)
