Final answer:
The sentence indicating that Happy Feet has become used to captivity is "Happy Feet adapted well to his new environment," as it signifies a successful adjustment to a non-natural environment.
Step-by-step explanation:
The sentence from the article that suggests that Happy Feet may have become accustomed to living in captivity is "Happy Feet adapted well to his new environment." This indicates that Happy Feet was comfortable and managed to adjust to life in a controlled, human-managed setting, which is a departure from his natural, wild habitat. Consequently, this aligns with the concept of an adaptation, which in evolutionary biology, refers to a heritable trait that improves an organism's survival and reproduction in its environment.