Which of the following diagrams best illustrates the relationship between Earth, Sea and Sun?

Let's analyze the relationship between the three entities logically:
1. Sea and Earth: Entirety of the Sea is situated on the Earth. Therefore, the category 'Sea' is completely contained inside the category 'Earth' ($\text{Sea}\subset \text{Earth}$).
2. Sun: The Sun is a completely independent star and is entirely separate from both Earth and Sea ($\text{Sun}\cap \text{Earth}=\varnothing$).

Thus, the ideal Venn diagram consists of a smaller circle (Sea) inside a larger circle (Earth), and a separate independent circle completely outside them (Sun). This configuration matches option (D).