IMO EConclusion: Small retailers that are forced to compete with large discount chains must offer exceptional service in order to retain their level of profitability
(A) not all large discount chains do in fact make a profit
May be, but we are concerned about the effect of large chains (that make profit) on small retailers
(B) some large discount chains have lower profit margins than do some small retailers
Ok, so that is supporting reasoning mentioned above somewhat. that means exceptional service is a reason
(C) small retailers are often motivated by things other than the desire for profit
Ok, but they still have to retain level of profitability.
(D) not all small retailers are forced to compete with large discount chains
But we are concerned about small retailers who are forced to compete
(E) exceptional service is not the only reason customers prefer small retail stores
That looks fine. So there are other factor however author reasoning is "must offer". Even if he don't offer there are other factor to cover up