A primary natural source of radon is

• A. Rocks and soils
• B. Ocean water
• C. Plants
• D. Building insulation

Answer: (A)

Radon comes from the radioactive decay of uranium in rocks and soils. In the Earth's crust, minerals containing uranium slowly decay and produce radon gas, which, being a gas, escapes from the solid phase into the pore spaces of soil and rock. From there it moves through the ground and can seep into buildings through cracks and openings, especially in basements and lower floors. Because soils and rocks continually generate radon, they’re the primary natural source of this gas for indoor air.

Ocean water can carry some dissolved radon, but it doesn’t contribute as a primary source to indoor radon levels. Plants don’t produce or release meaningful amounts of radon, and building insulation isn’t a source of radon; it can influence how much radon accumulates by affecting ventilation, but the gas originates mainly from the soil and rock beneath.