Final answer:
Individual preferences are affected by both internal factors like genetics and external influences such as environment and experiences, pointing to the conclusion that they are influenced by external factors.
Step-by-step explanation:
When we say that individual preferences are partly endogenous, it means that they are influenced by internal processes which can include genetic factors, but also by external influences such as the environment and personal experiences. Option B, "They are influenced by external factors," is the answer that correctly reflects this meaning. Preferences are formed through a complex interplay between an individual's biological makeup and the experiences they encounter throughout life, which includes cultural, social, and environmental factors.