Nested loop join Algo: https://en.wikipedia.org/wiki/Nested_loop_join
No. of block transfers, N(B) = $n_r*b_s+b_r$ ; when R is in outer loop and S is in inner loop
Where, $n_r$ are number of tuples in relation R.
$b_s$ and $b_r$ are number of blocks in relation S and R respectively.
size(r(R))<size(s(S)) blocks occupied by R are less ($b_r<b_s$). Also, ($n_r<n_s$). As, less no of blocks acquire less no. of rows.
For minimizing N(B), product term must be minimum and it will be minimum when R is in the outer loop.
For Example: let $b_r=10 , b_s=20; n_r=50, n_s=100$ since size(r(R))<size(s(S))
1. when R is in the Outer loop: N(B) = $n_r*b_s+b_r$ = 50*20+10 = 1010.
2. when S is in the outer loop: N(B) = $n_s*b_r+b_s$ = 100*10+20 = 1020.
So, relation R should be in outer loop.
Option(A).