Final answer:
The weak physical cross-link side bond between polypeptide chains that is disrupted by changes in pH is a hydrogen bond, and monosaccharides are linked together to form polysaccharides via glycosidic bonds.
Step-by-step explanation:
A weak, physical, cross-link side bond between adjacent polypeptide chains that is broken by changes in pH is a hydrogen bond. Hydrogen bonds are important non-covalent interactions that occur between polar amino acid side chains due to their affinity for highly electronegative atoms like oxygen or nitrogen. These bonds are relatively weak and can be disrupted by changes in pH, hence, among the options provided (A. Hydrogen, B. Carbon, C. Salt, D. Sulfur), the correct answer is A. Hydrogen.
Regarding monosaccharides forming polysaccharides, they are connected via glycosidic bonds. A glycosidic bond is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which could also be a carbohydrate.