When creating matching primary keys and foreign keys, which one is NOT a referential integrity rule?

When deleting the primary key, forbid it from being deleted when there are foreign keys that depend on it

When deleting the primary key, the foreign keys that dependent on it are automatically set to null.

When deleting the primary key, the foreign keys that dependent on it are also automatically deleted.

When deleting the primary key, the foreign keys that dependent on it automatically take the value of another primary key.