If a 1:M Relationship Is Optional on the 1 Side

If a 1:M relationship is optional on the 1 side and mandatory on the M side,which of the following is true?

A) The resulting foreign key can have null (empty) values
B) The resulting foreign key cannot have null (empty) values
C) The resulting foreign key must have null (empty) values
D) No foreign key is created as a result of mapping such a relationship