In darwinian terms, who are the fit? a. those who reproduce and replace themselves b. those who survive c. those who cooperate with others d. the most powerful

The correct answer here is A.
The fittest according in biological terms of  Charles Darwin and his evolutionary theory means the species which is able to reproduce and replace themselves. Fittest therefore is the species which, during the struggles of life manages to leave most of its offspring in the generations to come. The term was was first used by Herbert Spencer who saw parallels with his economic theory. He comperes it to Darwin's natural selection and Herbert States that it means the preservation and the survival of the favored species.