I have a doubt, How many rows will be returned when All values of R2.A will be Null….in this case number of rows Natural Join should give 0 rows ….what will be minimum number of rows???

I think you’re correct. In fact it R2.A need not even be NULL, any non NULL values such that $R_2.A \cap R_1.A = \phi$ should be sufficient to get zero rows in my opinion.