Necessary, because if it were not true — if civilized society can exist even if there are NO people who are willing to ignore their own welfare to help others — then we have no reason to believe that civilized society requires empathy. (A) describes the link between the premise and the new concept in the conclusion (what “civilized society” requires).

The author does believe that lack of empathy would result in people unwilling to follow certain moral codes. But that doesn’t commit the author to a belief that people will “usually” perform actions that harm civilized society. Perhaps they will perform those actions occasionally, but it won’t rise to the level of “usual.” In addition, it’s not clear that failure to follow moral codes involves actions that harm society.

The conclusion asserts what is required to have a civilized society. But at no point does the author try to argue that anything is sufficient for a society to be civilized.

The argument refers to moral codes that “sometimes” require people to ignore their own welfare to help others. That’s different from moral codes that “(always) require people to disregard their own welfare to help others.” The latter kind of code is more extreme than the codes the stimulus refers to. In any case, even if you think (D) refers to the same kind of moral codes as those in the stimulus, it’s still not necessary, because the author doesn’t have to believe that these codes have “arisen” in civilized societies. It just has to be the case that civilized societies require willingness to follow that kind of moral code. But where the moral code “arose” is irrelevant.

Although we know empathy is required to follow certain moral codes that sometimes involve ignoring one’s own welfare, that doesn’t imply that people who feel empathy “tend” to ignore their own welfare. They may sometimes ignore their own welfare to help others, but that doesn’t have to rise to the level of a general tendency to ignore one’s own welfare.