A nutritionist studying the effects of massive doses of vitamin C foun

Quite a bit of confusion on this one... let's see if we can sort it out.

Read the question stem: Which of the following hypotheses is best supported by the evidence above?

The use of the word 'Hypotheses' signifies that what we are looking for is a proposition that is highly probable in the light of the data given in stimulus.
The stimulus provides evidence (premises) for one of the answer options. So that answer option must be a hypothesis that is supported by the stimulus.

Premises:
- massive doses of vitamin C found that of a group of 600 people who regularly took 1,500 mg of vitamin C daily for a year, fewer than 9 percent suffered serious cases of flu;
- of a group of 600 people who took 250 mg of vitamin C (the standard recommended daily allowance) daily for a year, 34 percent suffered at least one serious case of flu;
- of a group of 600 people who took no vitamin C for a year (other than that found in the foods in a balanced diet), 32 percent suffered at least one serious case of flu.

Let's look at the options to see which one is supported by this data.

(A) The effectiveness of vitamin C in preventing serious cases of flu increases in direct proportion to the amount of vitamin C taken.
Not supported by the study. We do not know how much vitamin C was taken by people who took a balanced diet only. Anyway, we cannot establish the direct proportion.

(B) Vitamin C is helpful in preventing disease.
Too generic. Talks of disease in general. Not supported by the study.

(C) Doses of vitamin C that exceed the standard recommended daily allowance by 500 percent will reduce the incidence of serious cases of flu by 25 percent.
The popular but incorrect option. There is a difference between 'by 25%' and 'by 25 percent points'. If incidence of serious cases of flu was 34% and it reduces to 9%, that is a reduction of (34-9)/34 = 73.5% in the incidence. Or I can say that the incidence of serious flu has reduced by 25 percent points.
Similarly from 5%, if the growth rate goes up to 6%, that is an increase of 1/5 *100 = 20% (not 1%)
But I can say that the growth rate has increased by 1 percent point.

(D) Massive doses of vitamin C can help to prevent serious case of flu.
This is the hypothesis that is best supported by the study. Please note that it is not a 'must be true' statement. The author has already mentioned that these are hypotheses. We need to pick the one that is best supported by the stimulus. It is clear what massive doses are since the premise mentions "studying the effects of massive doses of vitamin C found that of a group...". The results of the study imply that massive doses of vitamin C can help prevent serious case of flu.

(E) A balanced diet contains less than 250 mg of vitamin C.
No information about this. If anything, in the group taking a balanced diet, the incidence of serious flu was slightly less than those taking 250 mg. So if we do want to infer something, we might be tempted to infer (wrongly, let me add) that a balanced diet has more than 250 mg of Vitamin C. We also don't know what kind of diet was taken by people taking 250 mg of Vitamin C.

Answer (D).