The possible answers are:
A. Variation in length of the amino acid backbone
B. Each protein is encoded by a distinct gene
C. Variations in the type of peptide bond
D. Variations in the glycosidic linkage
E. Variation in tertiary structure
F. Variations in which amino acids are used
The correct answers are A,E and F
There are only 20 amino acids that make all of the proteins in our bodies. However, there are options for protein variation are almost unlimited.
Firstly, you can vary the number of different amino acid that you use to make a protein.
Secondly, you can also vary the length of the amino acid chain.
And thirdly, when an amino acid chain is formed different parts of the chain interact with each other, bonding chemically, forming different 3-dimensional structures of the protein.
All of this contributes to the vast variation in proteins.