Let us see two scenarios with small number of tuples r1 having 5 tuples and r2 with 3 tuples.
Scenario 1: when all the attributes in R in r1 do not match value of R in r2.
natural join on r1 and r2 would give me
There are no uncommon values for R between r1 and r2
Natural join would give
So in this case we get the max number of tuples ,which is equal to the max number of tuples in relation r1.
Generalizing this inference we can derive that ans is a)2000 =max no of tuples in r1.