Questions: The level of protein structure that involves the interactions of multiple polypeptide strands to form more complex structures.

The answer is the second one: Quaternary.

Explanation for each option:

• Tertiary: This level of protein structure refers to the three-dimensional folding of a single polypeptide chain, resulting from interactions between the side chains (R groups) of the amino acids. It does not involve multiple polypeptide strands.

• Quaternary: This level of protein structure involves the assembly and interaction of multiple polypeptide chains (subunits) to form a functional protein complex. This is the correct answer as it specifically addresses the interaction of multiple polypeptide strands.

• Primary: This level refers to the linear sequence of amino acids in a polypeptide chain. It does not involve interactions between multiple strands.

• Secondary: This level involves local folding patterns within a polypeptide chain, such as alpha helices and beta sheets, stabilized by hydrogen bonds. It does not involve multiple polypeptide strands.

In summary, the quaternary structure is the level of protein structure that involves the interactions of multiple polypeptide strands to form more complex structures.