R(A,B,C) contains 5000 tuples, sorted on attribute A.
S(B,D) contains 100000 tuples, sorted on attribute B.
we have to do R join S, but join should be on some condition, otherwise it will simply Cartesian Product.
But by seeing the options, it will indicate that join means natural join.===> Attribute B is compared.
R.B == S.B ? already S is sorted on B ===> Sparse Index on S.B
But R is sorted on A ===> secondary index on R with B as key ===> Dense Index on R.B
(This is a non-standard question, no need to bother if more than one option is correct.)