Final answer:
The regions observed on a Ramachandran plot are the alpha helix and beta sheet regions.
Step-by-step explanation:
The regions that are observed on a Ramachandran plot are the alpha helix and beta sheet regions. Therefore, the correct answer to the question is a) Alpha helix and beta sheet regions. The Ramachandran plot is a way to visualize dihedral angles φ against ψ of amino acid residues in protein structure. The plot demonstrates the possible conformations of peptide chains, and notable regions on the plot correspond to the secondary structures of proteins, predominantly alpha helices and beta sheets. The plot emphasizes the steric constraints imposed by the side chain and backbone interactions, dictating which conformations are allowed for the residues within protein structures.