Final answer:
The term for assuming characteristics of an earlier form, created by adding one letter to a Darwinian word, is 'Atavistic'.
Step-by-step explanation:
Adding one letter to a Darwinian word results in a term meaning assuming characteristics of an earlier form. The term is b) Atavistic. This refers to an organism reverting to an ancestral trait or behavior, which is often seen as a throwback to a trait from an earlier stage of evolutionary history. This term is distinct from 'retrograde,' which typically means moving backward or reverting to an earlier or less advanced state or form.