Critical Reasoning

In an attempt to explain the cause of malaria, a deadly infectious disease common in tropical areas, early european settlers in Hong Kong attributed the malady to poisonous gases supposedly emanating from low-lying swampland. Malaria, in fact, translates from the Italian as "bad air." In the 1880s, however, doctors determined that Anopheles mosquitoes were responsible for transmitting the disease to humans after observing that the female of the species can carry a parasitic protozoan that is passed on to unsuspecting humans when a mosquito feasts on a person's blood.

Which function does the statement in boldface fulfill with respect to the argument presented above?

(A) It provides support for the explanation of a particular phenomenon.
(B) It presents evidence which contradicts an established fact.
(C) It offers confirmation of a contested assumption.
(D) It identifies the cause of an erroneous conclusion.
(E) It proposes a new conclusion in place of an earlier conjecture.

The OA is A.

I need help here. Where are the mistakes in options B, C, D and E.