With that being said, all of the remaining answer choices are POSSIBLE. You can have hydrogen bonding between polar R groups, hydrophobic interactions among nonpolar R groups, and also disulfide bonds, which form when two cysteine molecules somewhere in the chain covalently cross-link their thiol groups. Van Der Waals interactions, which are referring to the sum of attraction/repulsive forces that are exclusive of covalent bonds are also possible, but notice how this is just a clever restatement of answer choices B and D. 

With the definitions out of the way, we can now think about the strength of each of these forces. Specifically, B, D, and E are all INTER-molecular forces, while a covalent bond (choice C) is an INTRA-molecular force. If you recall from general chemistry, intermolecular forces are WEAKER than intramolecular forces, and thus C is the correct answer.