Final answer:
Jupiter and Saturn are often referred to as gas giants, but they have cores of rock and ice surrounded by thick layers of hydrogen and helium. The hydrogen is so compressed that it forms a layer of liquid as well as gas, making these planets more accurately described as having small, dense cores with thick surrounding gas layers.
Step-by-step explanation:
The structure of planets like Jupiter and Saturn most accurately fits the description of a small rock surrounded by a thick layer of gas (option d). However, it's important to clarify that both of these planets also have a significant layer of compressed, liquefied hydrogen. Under the force of gravity, the heavier elements including rock, metal, and ice form the cores of these planets. The cores are relatively small compared to the total mass of the planets, constituting only a few percent of the total mass. Surrounding these dense cores are extensive mantles of liquid hydrogen, forming under high pressure. The rest of the mass is primarily composed of hydrogen and helium gases, with the hydrogen being at such pressure to become liquid. This information indicates that although Jupiter and Saturn are often referred to as gas giants, they also contain liquid and solid components within their structures.