What the cartesian product actually doing is, it taking the studentId of "female" student from studInfo relation, and performing the cartesian product with all courses that are available... so it will return the all possible combination of "female" student can enroll in various courses.. Now finally we are subtracting "enroll" from it.. "enroll" relation contain what actually enrollment has been done.. so the substraction will return the "courseId" a "female student has not been enrolled..!!
because, ( all possible enrollment in various course of female) - ( actual enrollment) = courses in which female enrollment has not happened!!...
Option (b) is saying "female student which are enrolled (proper Subset)".. No but it is returning courses in which female student are NOT enrolled!!.. so option (D)