Option (A).
Consider the relations:
STudent
--------------------------------------
1 sush 21
2 sush 22
3 Jaya 23
courses
--------------------------
11 Java
22 OS
Enroll
---------------------------
1 11
2 22
3 11
3 22
Now, query Q2 gives $\phi$ for the above relations.Q1 returns 'Jaya'. In fact, the problem with query Q2 is even if someone hasnt registered for any course, his name will be displayed.