Final answer:
Interactionism is the theory that states nature and nurture influence each other, highlighting the gene-environment interactions that shape an individual.
Step-by-step explanation:
The theory that posits both nature and nurture have an influence on each other is known as interactionism. It emphasizes that humans are shaped by the interplay between biological factors, such as our genetics and physiological traits, and environmental factors, including our upbringing, culture, and personal experiences. This notion is encapsulated in the concept of gene-environment interactions, asserting that our genetic potential is expressed within the context of our environmental experiences, which in turn can be influenced by our genetic makeup. Hence, the correct answer to the student's question is b) Interactionism.