Which integrity constraint is violated in this relation definition?

Consider the following relation definition:

STUDENT(
Student_Number: integer NOT NULL
Name: variable length character string length 20 NOT NULL) Primary Key Student_Number
HOUSING(
Housing_ID: integer NOT NULL
Student_Number: integer NOT NULL
Building: variable length character string length 25 NOT NULL) Primary Key Housing_ID
Foreign Key Student_Number References
STUDENT(Student_Number)
ON DELETE NO CHECK
ON UPDATE

Which integrity constraint is violated in this relation definition?

A.
Entity integrity

B.
Domain constraint

C.
Referential integrity

D.
Enterprise constraint