The Miller-Urey experiment demonstrated that under plausible early-Earth conditions, spontaneous assembly of what type of molecules from inorganic precursors can occur?

• A. Direct synthesis of DNA
• B. Spontaneous assembly of organic molecules from inorganic molecules
• C. Formation of complex proteins
• D. Synthesis of amino acids only

Answer: (B)

Abiotic synthesis of organic compounds from inorganic starting materials under conditions that resemble early Earth.

The Miller-Urey setup showed that energizing a mixture of simple inorganic gases with water—to mimic lightning and volcanic activity—produces small organic molecules, including amino acids. This demonstrates that the building blocks of life can form spontaneously without living chemistry, which is the key idea about the origin of life being possible from non-biological starting points.

The other options aren’t as accurate because the experiment did not create DNA directly, nor complex proteins that require cellular machinery, and it isn’t limited to amino acids alone—it's about forming organic molecules in general from inorganic precursors.