First it selects all the female students.
Next it takes the studids of all the female students present.
Then it cross with all the courseid s present, which creates extra records which are the tuples where some female students not enrolled for some courses.
When we set difference the two sets, the extra records that is the tuples, in which, course IDs are presentwhich are not enrolled by some-some females.
Here is the catch - if some course is not enrolled by some female, it may have been enrolled by some other female as it is mentioned in the question that at least one male and female for each course must be enrolled.
Therefore we can understand the course IDs which we will get as result is enrolled by some female which is the proper subset of females, also it can be 'fi' if all females enrolled for all courses.
So, Option B is correct