Consider the following protein chain:

How many amino acids make the atoms in a planar unit? Why are the units planar?

Only one amino acid makes the atoms in the planar units. The units are planar because the double bond from the carbonyl group (C=O) makes rotation around this bond restricted.

Two amino acids make the atoms in the planar units. The units are planar because the electrons are delocalized throughout the C=O and C–N bonds, restricting rotation around that bond.

Only one amino acid makes the atoms in the planar units. The units are planar because amino acid side chains will restrict the rotation of the bonds around the central carbon atom.

Two amino acids make the atoms in the planar units. The units are planar because the amide bond (C–N) is from two amino acids, restricting rotation around that bond.