Final answer:
Earth is the best planet for building life molecules of CHONPS due to its water availability and robust biosphere; Mars also demonstrates potential for past habitable conditions.
Step-by-step explanation:
The best planet for building molecules of life containing carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHONPS) would be Earth. This is because Earth hosts a robust biosphere with the presence of organic molecules and the availability of liquid water, which is known to be a critical requirement for life as we understand it. The exploration of Mars has also revealed the potential for past habitable conditions and presence of organic chemicals, making it a key target for the search for evidence of life. However, Earth's conditions are currently the most conducive to supporting life that involves such complex molecules. Jupiter, being a gas giant, lacks a solid surface and supportive environment for life-building molecules to form and persist. Venus, with its extreme surface conditions and harsh atmosphere, is less likely to support Earth-like life.