Final answer:
Neptune has a small internal energy source and is the only planet among the options given that is still undergoing internal differentiation.
Step-by-step explanation:
The question asks which of the following planets has an interior that is still differentiating: Earth, Saturn, Uranus, or Neptune. Among the choices, Neptune is noteworthy for having a small internal energy source, allowing its interior to undergo differentiation. Jupiter, Saturn, and Neptune are known to have major internal heat sources, which contribute to their dynamism and differentiation. In comparison, Uranus lacks a measurable internal heat source, suggesting that its interior is not actively differentiating. Therefore, the correct answer is D) Neptune.