I picked C. The way I interpreted it was very similar to what B was saying. An effect (back pain) that occurs in the absence of a particular phenomenon (slipped disk) might not occur when that phenomenon is present. In other words: back pain may not occur when a slipped disk is present. To me, it was a very hard choice between B and C, because the way I interpreted it seemed to say almost the same thing to me.

where I am having trouble is in figuring out where the sufficiency indicator is on the stimulus.