Question

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

Explanation:

Producers (plants) can absorb nitrogen atoms in the soil (in the form of nitrates etc.). Carbon atom in a cow is part of an animal (consumer), sulfur in air is not in a form readily taken up by producers (they take up sulfur from soil in some forms like sulfate but not as free sulfur atom in air in significant amounts for this context), iron in rock is not in a bio - available form for producers to directly pick up.

Answer:

a nitrogen atom in the soil