Answer:
Unconditioned response.
Explanation:
Classical conditioning may be defined as the learning procedure when the biological strong stimulus is combined with neutral stimulus elicit a strong response.
In case of Carla the nausea from the chemotherapy acts as unconditioned response. The nausea is unlearned response and occurs due to the regular chemotherapy. The chemotherapy acts as unconditioned stimulus that results in the nausea that acts as unconditioned response.
Thus, the correct answer is option (d).