Question

which atom is available to be picked up by producers?
a sulfur atom in the air
a carbon atom in a cow
an iron atom in a rock
a nitrogen atom in the soil

Explanation:

Producers (like plants) absorb nutrients from the soil. Nitrogen in soil (often as nitrates) is a key nutrient plants take up. Sulfur in air is not readily available, carbon in a cow is part of a consumer, and iron in rock is not accessible to producers without weathering. Thus, the nitrogen atom in soil is available.

Answer:

a nitrogen atom in the soil