Question

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

Explanation:

Producers (like plants) can take up nitrogen atoms from the soil (in the form of nitrates etc.). A carbon atom in a cow is part of an animal (not available directly as such for producers in this form). An iron atom in a rock is not in a form readily taken up by producers. A sulfur atom in the air is not in a form (like sulfate in soil) that producers can directly take up. Producers mainly take up nutrients from the soil (for nitrogen, it's often in soil - available forms like nitrates).

Answer:

a nitrogen atom in the soil