Final answer:
The rock that recorded the appearance of free oxygen in the atmosphere is the banded iron formation.
Step-by-step explanation:
The rock that recorded the appearance of free oxygen in the atmosphere is the banded iron formation. Banded iron formations are sedimentary rocks composed of alternating bands of iron-rich minerals, such as hematite and magnetite, and iron-poor minerals, such as shale or chert. These formations indicate the presence of oxygen because hematite and magnetite can only form in the presence of high concentrations of dissolved oxygen.