Focus on "instead" in the sentence.

Why isnt it C,F? As we can think that the Theories that could verify scientific objectivity have turned out the be biased, Then Impartial science takes a position that a lot of the knowledge areas are human construct.

When theories formerly considered to be (i) _______ in their scientific objectivity are found instead to reflect a consistent observational and evaluative bias, then the presumed neutrality of science gives way to the recognition that categories of knowledge are human (ii) _______ .

The phrase "are found instead" creates a contrast between "reflect a consistent observational and evaluative bias" and "theories formerly considered to be (i) _______ in their scientific objectivity".

Thus, the theories ought to be disinterested in their scientific objectivity to contrast with the subjectivity (observational and evaluative bias) mentioned later.

Verifiable is not the right word, because we are trying to measure the theories in the parameter of scientific objectivity. Either it is interested (biased) or disinterested. Neither does callous make sense here.

Now if the presumed neutrality gives way, then categories of knowledge become subjective or recognized as human constructions.