The summation is for each node, if that node happens to be the root. When a node is root, it will have $(k-1)$ nodes on the left sub tree ($k$ being any number) and correspondingly $(n-k)$ elements on the right sub tree. So, we can write recurrence $T(k-1) * T(n-k) $ for the number of distinct binary search trees, as the numbers on left and right sub trees form BSTs independent of each other and only a difference in one of the sub trees produces a difference in the tree. Hence, answer is B.
Knowing the direct formula can also help in getting the answer but is not recommended.
https://gatecse.in/number-of-binary-trees-possible-with-n-nodes/