Darwin's ideas espoused in both Origin of Species and The Descent of Man were incompatible with the prevalent scientific racism of the age (like that of Robert Knox, for example) because of Darwin's emphasis on .

The correct answer is "change".

Explanation:

The missing options of this question are:

A. natural selection.

B. immutability.

C. survival of the fittest.

D. change.

The correct answer is option D. "change".

The scientific racism that was prevalent during Charles Darwin books publications was related to the concept of "pure races", and the association of the inheritance of the "superior attributes" related to these races. Darwin's ideas were incompatible to this scientific racism, since Charle Darwin theories support the idea of a constant change among all people attributes, produced by genetics behavior and exposure to environmental factors.