C is the answer to this question. This can be related to the primary key but this time, when you click on the foreign key, it will refer you to the primary table. One table may have more than one foreign keys. It will depend on the data that will be placed on the table. Each foreign key will be placed differently by the database system.
There are also instances when a foreign key can be used to refer to a constraint that is unique in nature. This will always depend on the person who is managing and handling the database on how foreign keys can be used and how many foreign keys can be available for every primary table.