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A protein's biological activity is primarily determined by its three-dimensional structure. The correct answer is that proteins must achieve a specific folded structure known as tertiary structure to function properly.

The tertiary structure refers to the overall three-dimensional arrangement of all the amino acid chains within a single polypeptide. This level of structure is crucial because it dictates how a protein interacts with other molecules, including substrates, inhibitors, and other proteins. In many cases, the biological activity of enzymes and receptors is directly tied to their tertiary structure. A change in the folding pattern can lead to a loss of function or altered activity.

While secondary structures like alpha helices and beta sheets contribute to the protein's overall folding, they are not sufficient alone for biological function. The quaternary structure becomes important for proteins that consist of multiple polypeptide chains, but it is the tertiary structure that primarily determines the function of the individual polypeptide chains. The primary structure, which simply refers to the sequence of amino acids, is foundational but does not indicate the folding needed for activity. Thus, the tertiary structure is essential for a protein to be biologically active.