Final answer:
The protein level that best matches the description "blocks on a string" is the primary structure.
Step-by-step explanation:
The structural level of proteins that best matches the description "blocks on a string" is the primary structure.
The primary structure of a protein refers to the sequence of amino acids that make up the polypeptide chain. It is like a string of building blocks, with each amino acid representing a block.
The secondary structure (such as alpha-helix or beta-pleated sheet), tertiary structure (overall 3D folding), and quaternary structure (interactions between multiple subunits) are higher levels of protein structure, but they do not match the description provided.