9. suppose that species a evolves and species b evolves in response to the changes in species a. species a then changes in response to the changes in species b. this is an example of

a. convergent evolution.
b. extinction.
c. adaptive radiation.
d. coevolution.

Correct D). Co-evolution

The co-evolution is the process of reciprocal revolutionary changes that are known to occur between the two or more species. The activity of each species, which participate in the interaction, applies a selection pressure to the others. It is one of the primary methods by which biological communities are organized.  

Example: Predator-prey relationship, in this relationship, there is selective pressure on prey in order to avoid capture. As a result of that, the predator must adapt to become a more effective hunter.

Hence, the correct answer would be option A.