Final answer:
The best description of conditions on early Earth is the presence of very low levels of free oxygen, which allowed for complex chemical reactions necessary for life's building blocks.
Step-by-step explanation:
The conditions scientists hypothesize to have existed on early Earth can be best described by the presence of very low levels of free oxygen. Early Earth's atmosphere is known to have contained abundant carbon dioxide and methane, but it lacked oxygen gas, which is a byproduct of photosynthesis and would only accumulate after photosynthetic organisms evolved.
These conditions allowed for complex chemical reactions that are not possible in an oxygen-rich environment, leading to the production of amino acids, proteins, and other chemical building blocks of life.