Scientists must be impartial and not base their conclusions on
a. opinions
c. models
b. evidence
d. experiments

Scientists should definitely use evidence - and they collect this evidence though especially designed experiments. Based on their data and assumptions they build models which they then test, but they can base their conclusions based on the models too.

What they should not do is based the conclusions on opinions, because opinions are not imparcial- they can state them as opinions, but not as conclusions.

Smelly rocks sink in water haha!