Final answer:
The 5th planet from the Sun, primarily composed of hydrogen and helium, is Jupiter. It is a gas giant unlike the terrestrial planets Venus, Earth, and Mars, and no one has walked on it.
Step-by-step explanation:
The question refers to the 5th planet from the Sun which is primarily composed of hydrogen and helium. The correct answer is (c) Jupiter, as it matches this description. Venus (a) and Mars (d) are terrestrial planets with solid surfaces and rocky compositions, while Earth (b) is also a terrestrial planet and the only one known to support life. Jupiter, in contrast, is a gas giant and the largest planet in our solar system, consisting mostly of hydrogen and helium, making it vastly different from the inner terrestrial planets.
Venus and Mars are of interest to space exploration, but they have conditions that are not similar to Earth's. Venus is extremely hot and has a thick atmosphere dominated by carbon dioxide, while Mars is much colder and has a thin atmosphere. Earth is the only one among them with a hospitable climate that can support life. Jupiter, being a gas giant, has a fundamentally different composition and conditions compared to Venus, Earth, and Mars.