The passage indicates that:
A deficiency in all B-vitamins was known to cause beriberi.
A deficiency in B1 (vitamin B1, or thiamine) is always associated with beriberi.
Recent experiments showed that human volunteers deficient only in B1 did not contract beriberi.
This contradiction suggests that B1 deficiency alone is insufficient to cause beriberi; other B-vitamins are likely necessary as well.
D resolves the contradiction by explaining that beriberi is caused by deficiencies in a combination of B-vitamins, not just B1. This aligns with the earlier finding that being deficient in all B-vitamins causes beriberi.
Why not the other options?
A: This introduces a new idea about a generally unhealthy diet, which is outside the scope of the question and not mentioned in the passage.
B: This contradicts the information given; B1 deficiency is necessary but not sufficient alone to cause beriberi.
C: This discusses a cure, not the cause of beriberi.
E: This is too broad and unrelated to the specific experiment mentioned.