ANS:D - foreign keys.

If any set of attributes of one table behaves as primary keys in another tables then that set of attributes are foreign keys w.r.t the another tables. Always remember one table can contain only one primary key which is set by DBA (Database administrator). So according to my view it may be that there should not be more than one foreign key in any table/relation.

Since, the relationship set makes the relation among the different entity sets so for the relationship table there must be some fields which reference the different entity tables one by one and also some different fields. It means that some fields of the relationship set acts as the primary keys in the different entity sets so these fields are the foreign keys w.r.t. The tables which is referenced by that fields of the relationship tables. So it is clear that,

In the relational model, relationships between relations or tables are created by using:

Answer: Foreign keys.