Explanation : It is 12. The tree may be of depth 2 or 1. if the depth is 2, we have 6 possible trees. This is because one of the three nodes A, B, C may be the root and the next level may be one of the remaining two nodes. If the depth is 1, the root may be one of the 3 nodes A, B, C. Corresponding to a root, say A, two trees as possible as this.