The proteins are formed of the monomer units called amino acids which bond with each other via peptide bond and form a linear peptide structure called the primary level structure of the protein.
The proteins to perform several functions in a cell undergoes structural conformation and attain the helical form called alpha helix due to the involvement of alpha carbon in bond formation and a pleated sheet called beta-pleated sheet due to the involvement of beta carbon. These structures are known as a secondary level of protein structure.
Thus, a Secondary level of protein structure is the correct answer.
Proteins are complex molecules that consist of small chains of amino acids joined together found in human body. Large chains are formed by the buildup of small chains to form complex structure. Alpha helix or beta pleated sheet are examples of secondary level of protein structure. This type of structure occur in the polypeptide which is due to interactions between amino hydrogen atoms and carboxyl atoms in the peptide backbone. The secondary structure level is the intermediate before the protein form three dimensional tertiary structure of protein. We have three structure Level of protein: primary, secondary and tertiary structure level of protein.
The best answer is D, birds. For more information and details on the way a bird's respiratory system works: https://asknature.org/strategy/air-flow-patterns-facilitate-efficient-gas-exchange/
