It has long been noted that there is a much higher incidence

I don't agree with OA. D for me.

The argument above about the dangers of moderate drinking assumes that

A) alcoholics probably cause damage to other organs of the body as well as to their livers
Author is specifically concerned with effects of moderate drinking on the liver. Could have been 2nd best answer.

B) one glass of wine before dinner is not considered harmful
Irrelevant

C) there is little difference between moderate and heavy drinking in terms of damage to internal organs
The author does not assume this. This is an opinion stated by the medical researchers.

D) the effects of moderate drinking on human and experimental animals are similar
The author uses experimental tests done on rats to make his point about the dangers of moderate drinking. In other words he assumes that the effects of moderate drinking on human and experimental animals are similar. Correct answer for me.

E) people often enjoy a lifetime of moderate drinking without apparent effects
Irrelevant.