Hi Lydia,
For this question you need to determine whether there is intra or intermolecular forces, and determine whether there is hydrogen bonding or covalent bonding.
We figure that the reaction is intermolecular because it is talking about multiple molecules coming together - intermolecular.
Now we need to know whether there is hydrogen or covalent bonding. We conclude that it is covalent because of the hints that there is disulfide bonding.
You are correct that most often covalent bonds are intramolecular, but there are occasions where it can be inter molecular - like disulfide bonds.