Personally, I find this question to be really retarded (and easy, but for other reasons).
However, in philosophy there is a marked difference between the use of the word selfish.
1. Caring for one's pleasure with noted disregard for other people.
2. Thoughts or actions that come from an individual. (personal, ethical definition)
In the latter case, selfish becomes nearly synonymous (practically speaking) with the word personal. The last sentence then becomes in effect:
"When the deeper implications are considered, even the simplest "unselfish" acts prove to be instances of (personal) concern for the human species."
Said that way, the difference is obvious. I've had the difference drilled into my head from phil of ethics courses though...I would imagine it would be tougher for others to recognize. However, I still think this question is very easy...the other answer choices are really dumb:
d) We are talking about human society, so who cares if other species arent mentioned? Irrelevant.
c) No it doesnt. Easily dismissed choice.
b) you dont need stats to put forward an argument, silly answer choice.
a) Of course it is...the claim of selfishness as always motivating human action is a core premise of defending that all unselfish acts are still selfish ones, with the assumption that human motivation hasn't changed (however fallacious that is).
There is no good second answer choice. That would make e) very obvious by the end.