Let us consider both the cases which needs to be addressed in this question :
a) All the tuples of Enroll relation contains different roll no.
In this case , exactly one tuple of student relation is going to be mapped with one tuple of enroll relation , so no of tuples as a result of natural join in this case = 16 as enroll relation contains 16 distinct values of roll no. and hence possible no of tuples in the natural join will be 16
b) All the tuples of Enroll relation contains same roll no.
In this case , exactly one of the tuples of student relation is going to combine with all 16 tuples of Enroll relation and no other tuple from Student relation will combine since their roll no values will be different.Hence the possible no of tuples in the natural join is going to be 16 here as well.Further we can take same value of roll no in enroll relation since (rollno , courseno) is the primary key , so different values of courseno can be taken to maintain the primary key integrity constraints.
Hence , in all cases we get the no of tuples in the natural join = 16
Hence , max = min no of tuples = 16
Hence , A) should be the correct option.
