Answer is B.
The maximum number of tuples can be $mn. $ This happens in two cases.
Case 1: if there is a common attribute between $R$ and $S$, and every row of $r$ matches with the each row of $s$- i.e., it means, the join attribute has the same value in all the rows of both $r$ and $s$,
Case 2: If there is no common attribute between $R$ and $S.$
The minimum number of tuples is $0. $ This happens when there is a common attribute between $R$ and $S$ and it has no common value between the two relations.