When an oxygen molecule becomes a free radical, it is referred to as a(n): ?

Tertiary Structure

Explanation:

Proteins are made up of amino acid units. The four types of protein structure are primary, secondary, tertiary, and quaternary.

From the given options, the greatest effect on a protein's function is caused by tertiary structure because it has branched structure and have several side chains. These side chains can interact in different ways and can change the effect on a protein's function.

Hence, the correct answer is "Tertiary Structure".