Option (a) A dense index on R.B
Here it is given that R is sorted on attribute A and S is sorted on attribute B. As R is already sorted on attribute A, so it may or may not be sorted with respect to attribute B. Hence secondary indexing(since it may be non-key or candidate key and the file may be unordered w.r.t B) has be present for R where the key field is R.B. As secondary indexing is dense indexing, hence a dense index needs to be formed on the attribute R.B.