Which of the following statements best describes the meaning of a referential integrity constraint in the relational data model?

a. The primary key of tuple in a relation must uniquely identify the tuple in the database

b. Users cannot reference a tuple of a relation unless they have been assigned the appropriate action privileges

c. The primary key value of a tuple of a relation cannot have a null value

d. If a data item in a tuple of one relation references the key of a tuple of another relation, the latter tuple must exist in the database