Question

quaternary structure describes
the formation of protein sheets.
the formation of helical shapes.
the combination of two different folded chains.
extensive branching.
a form of denaturing into a linear string of amino acids.

Explanation:

Protein secondary structure includes alpha helices (helical shapes) and beta sheets. Protein quaternary structure specifically refers to the arrangement of multiple folded polypeptide chains (subunits) that come together to form a single functional protein complex. Denaturation is the loss of protein structure, and extensive branching is not a feature of protein structure levels.

Answer:

the combination of two different folded chains.