Answer is C
Here in (i) when we update in STUDENT table Dept_id = NULL it is fine as a foreign key can be NULL.
But in (ii) if we set in DEPARTMENT table dept id = NULL it is not possible as PRIMARY KEY cannot be NULL.
Instead of update to NULL, if we try DELETE, then also it is not allowed as we have foreign key reference to it from STUDENT table with Dept_id = 1. DELETE ON CASCADE clause is a way to avoid this issue which will delete all referenced entries from the child table too but unless told we cannot assume this as this cause is not universally applicable.