The rule of union by rank is to attach the smallest tree to the root of the largest tree.
At the first stage, A merges with D (this is just an example, I think you could do it any other way), so after union(A, D) you can have either A_0 -> D_1 , or D_O -> A_1 , since 2 singleton trees have the same rank, which you randomly select, in this case D , to be the root.