The correct answer is a) tertiary.
Proteins are made up of long chains of amino acids that fold into specific three-dimensional shapes, known as their tertiary structure. This folding is crucial because the shape of a protein directly influences its function. The primary structure of a protein refers to the sequence of amino acids, but it is the way those amino acids arrange themselves into a three-dimensional structure (tertiary structure) that determines how the protein will interact with other molecules and perform its biological functions. Therefore, while all levels of structure (primary, secondary, and tertiary) are important in the overall makeup of a protein, it is the tertiary structure that is ultimately responsible for its functional capabilities.